DOC: update the DataFrame.applymap 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:

```
python scripts/validate_docstrings.py pandas.DataFrame.applymap

################################################################################
#################### Docstring (pandas.DataFrame.applymap)  ####################
################################################################################

Apply a function to a Dataframe elementwise.

This method applies a function to a DataFrame that is intended to
operate elementwise, i.e. like doing `map(func, series)` for each
series in the DataFrame.

Parameters
----------
func : function
    Python function, returns a single value from a single value.

Returns
-------
pandas.DataFrame
    A transformed DataFrame.

See also
--------
DataFrame.apply : Apply a function along input axis of DataFrame

Examples
--------
>>> df = pd.DataFrame([[1.,2.], [3.,4.]])
>>> df
     0    1
0  1.0  2.0
1  3.0  4.0
>>> df = df.applymap(lambda x: x**2)
>>> df
     0     1
0  1.0   4.0
1  9.0  16.0

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

Docstring for "pandas.DataFrame.applymap" correct. :)
```