DOC: update the series.dt.weekofyear docstring
Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

- [x] PR title is "DOC: update the <your-function-or-method> docstring"
- [x] The validation script passes: `scripts/validate_docstrings.py <your-function-or-method>`
- [x] The PEP8 style check passes: `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] The html version looks good: `python doc/make.py --single <your-function-or-method>`
- [x] It has been proofread on language by another sprint participant

Please include the output of the validation script below between the "```" ticks:

```
################################################################################
###################### Docstring (pandas.Series.dt.week)  ######################
################################################################################

The week ordinal of the year.

Return the week ordinal of the year. Note that there can be
counter intuitive edge cases around the yearchange. For example
the first of january could be in week 52 of the previous year.
Monday indicates the start of a new week.

See Also
--------
pandas.Series.dt.week : Identical method.
pandas.Series.dt.weekofyear : Identical method.

Returns
-------
ndarray of integers indicating week

Examples
--------
>>> dates = pd.date_range("2016-12-31", "2017-01-08", freq="D")
>>> s = pd.Series(dates)
>>> pd.DataFrame({'date':s, 'week':s.dt.week, 'day':s.dt.strftime("%A")})
        date  week        day
0 2016-12-31    52   Saturday
1 2017-01-01    52     Sunday
2 2017-01-02     1     Monday
3 2017-01-03     1    Tuesday
4 2017-01-04     1  Wednesday
5 2017-01-05     1   Thursday
6 2017-01-06     1     Friday
7 2017-01-07     1   Saturday
8 2017-01-08     1     Sunday

Note what `series.dt.week` and `series.dt.weekofyear` are the same.

>>> s.dt.week
0    52
1    52
2     1
3     1
4     1
5     1
6     1
7     1
8     1
dtype: int64
>>> s.dt.weekofyear
0    52
1    52
2     1
3     1
4     1
5     1
6     1
7     1
8     1
dtype: int64

################################################################################
################################## Validation ##################################
################################################################################

Docstring for "pandas.Series.dt.week" correct. :)
```

If the validation script still gives errors, but you think there is a good reason
to deviate in this case (and there are certainly such cases), please state this
explicitly.