DOC: update the MultiIndex.swaplevel 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.MultiIndex.swaplevel)  ###################
################################################################################

Swap level i with level j.

Calling this method does not change the ordering of anything.

Parameters
----------
i : int, str
    First level of index to be swapped. Can pass level name as string.
    Type of parameters can be mixed.
j : int, str
    Second level of index to be swapped. Can pass level name as string.
    Type of parameters can be mixed.

Returns
-------
MultiIndex
    A newly allocated MultiIndex

.. versionchanged:: 0.18.1

   The indexes ``i`` and ``j`` are now optional, and default to
   the two innermost levels of the index.

See Also
--------
MultiIndex : A multi-level, or hierarchical, index object for
    pandas objects.

Examples
--------
>>> mi = pd.MultiIndex(levels=[['a', 'b'], ['bb', 'aa']],
...                    labels=[[0, 0, 1, 1], [0, 1, 0, 1]])
>>> mi
MultiIndex(levels=[['a', 'b'], ['bb', 'aa']],
   labels=[[0, 0, 1, 1], [0, 1, 0, 1]])
>>> mi.swaplevel()
MultiIndex(levels=[['bb', 'aa'], ['a', 'b']],
   labels=[[0, 1, 0, 1], [0, 0, 1, 1]])

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

Docstring for "pandas.MultiIndex.swaplevel" correct. :
```