DOC: Fix indentation in docstring example

Not sure why we didn't see it before, but the example in the first code block of the docstring guidelines seem to be missing the indentation: https://pandas.io/docs/development/contributing_docstring.html#about-docstrings-and-standards

We should fix it, and indent the function properly: https://github.com/pandas-dev/pandas/blame/master/doc/source/development/contributing_docstring.rst#L25