DOC: Fix double blank lines in Series docstrings
In the docstrings, we use single blank lines to separate sections (or inside sections). For example:
```
Notes
------
This is a notes section.

Examples
---------
This is the examples section, there is just a blank line between the
end of the notes section and this one.

>>> print('only one blank line before the previous paragraph and this code')
only one blank line before the previous paragraph and this code
```

But in some cases two blank lines are incorrectly left:
```
Notes
------
This is a notes section.


Examples
---------
Now there are two blank lines before examples.


>>> print('and two blank lines before the code too')
and two blank lines before the code too
```

This happen somehow often, but we should remove the extra blank lines, to keep the format consistent. In this issue we'll fix only the docstrings in `Series` having this problem.

To get the list, this can be used (after fixing, the same command shouldn't report anything):
```
$ ./scripts/validate_docstrings.py --prefix=pandas.Series --errors=GL03
pandas.Series.values: Use only one blank line to separate sections or paragraphs
pandas.Series.xs: Use only one blank line to separate sections or paragraphs
pandas.Series.round: Use only one blank line to separate sections or paragraphs
pandas.Series.product: Use only one blank line to separate sections or paragraphs
pandas.Series.apply: Use only one blank line to separate sections or paragraphs
pandas.Series.rolling: Use only one blank line to separate sections or paragraphs
pandas.Series.corr: Use only one blank line to separate sections or paragraphs
pandas.Series.kurt: Use only one blank line to separate sections or paragraphs
pandas.Series.mad: Use only one blank line to separate sections or paragraphs
pandas.Series.max: Use only one blank line to separate sections or paragraphs
pandas.Series.mean: Use only one blank line to separate sections or paragraphs
pandas.Series.median: Use only one blank line to separate sections or paragraphs
pandas.Series.min: Use only one blank line to separate sections or paragraphs
pandas.Series.prod: Use only one blank line to separate sections or paragraphs
pandas.Series.sem: Use only one blank line to separate sections or paragraphs
pandas.Series.skew: Use only one blank line to separate sections or paragraphs
pandas.Series.std: Use only one blank line to separate sections or paragraphs
pandas.Series.sum: Use only one blank line to separate sections or paragraphs
pandas.Series.var: Use only one blank line to separate sections or paragraphs
pandas.Series.kurtosis: Use only one blank line to separate sections or paragraphs
pandas.Series.compound: Use only one blank line to separate sections or paragraphs
pandas.Series.droplevel: Use only one blank line to separate sections or paragraphs
pandas.Series.rename_axis: Use only one blank line to separate sections or paragraphs
pandas.Series.argmin: Use only one blank line to separate sections or paragraphs
pandas.Series.argmax: Use only one blank line to separate sections or paragraphs
pandas.Series.swaplevel: Use only one blank line to separate sections or paragraphs
pandas.Series.append: Use only one blank line to separate sections or paragraphs
pandas.Series.update: Use only one blank line to separate sections or paragraphs
pandas.Series.tz_localize: Use only one blank line to separate sections or paragraphs
pandas.Series.dt.tz_localize: Use only one blank line to separate sections or paragraphs
pandas.Series.str.findall: Use only one blank line to separate sections or paragraphs
pandas.Series.str.match: Use only one blank line to separate sections or paragraphs
pandas.Series.str.partition: Use only one blank line to separate sections or paragraphs
pandas.Series.str.replace: Use only one blank line to separate sections or paragraphs
pandas.Series.str.rpartition: Use only one blank line to separate sections or paragraphs
pandas.Series.cat: Use only one blank line to separate sections or paragraphs
pandas.Series.hist: Use only one blank line to separate sections or paragraphs
pandas.Series.to_excel: Use only one blank line to separate sections or paragraphs
pandas.Series.to_hdf: Use only one blank line to separate sections or paragraphs
pandas.Series.as_matrix: Use only one blank line to separate sections or paragraphs
pandas.Series.from_array: Use only one blank line to separate sections or paragraphs
pandas.Series.ix: Use only one blank line to separate sections or paragraphs
pandas.Series.ptp: Use only one blank line to separate sections or paragraphs
```