DOC: fix the pandas.DataFrame.add example
Docstring assigned to the PyData Nairobi chapter for the sprint
Checklist for the pandas documentation sprint 

- [x] PR title is "DOC: update the pandas.DataFrame.rmul docstring"
- [x] The validation script passes: `scripts/validate_docstrings.py pandas.DataFrame.rmul`
- [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 pandas.DataFrame.rmul`
- [ ] It has been proofread on language by another sprint participant

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

`scripts/validate_docstrings.py pandas.DataFrame.rmul`
```
################################################################################
###################### Docstring (pandas.DataFrame.rmul)  ######################
################################################################################

Multiplication of dataframe and other, element-wise (binary operator `rmul`).

Equivalent to ``other * dataframe``, but with support to substitute a fill_value for
missing data in one of the inputs.

Parameters
----------
other : Series, DataFrame, or constant
axis : {0, 1, 'index', 'columns'}
    For Series input, axis to match Series index on
level : int or name
    Broadcast across a level, matching Index values on the
    passed MultiIndex level
fill_value : None or float value, default None
    Fill existing missing (NaN) values, and any new element needed for
    successful DataFrame alignment, with this value before computation.
    If data in both corresponding DataFrame locations is missing
    the result will be missing

Notes
-----
Mismatched indices will be unioned together

Returns
-------
result : DataFrame

Examples
--------
>>> a = pd.DataFrame([1, 1, 1, np.nan], index=['a', 'b', 'c', 'd'],
...                  columns=['one'])
>>> a
   one
a  1.0
b  1.0
c  1.0
d  NaN
>>> b = pd.DataFrame(dict(one=[1, np.nan, 1, np.nan],
...                       two=[np.nan, 2, np.nan, 2]),
...                   index=['a', 'b', 'd', 'e'])
>>> b
   one  two
a  1.0  NaN
b  NaN  2.0
d  1.0  NaN
e  NaN  2.0
>>> a.add(b, fill_value=0)
   one  two
a  2.0  NaN
b  1.0  2.0
c  1.0  NaN
d  1.0  NaN
e  NaN  2.0

See also
--------
DataFrame.mul

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

Docstring for "pandas.DataFrame.rmul" 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.

