DOC: fix an example which raises an Error in whatsnew/v0.10.0.rst
Fix an example in doc/source/whatsnew/v0.10.0.rst, which raises an IndentationError (see 
https://pandas.pydata.org/docs/dev/whatsnew/v0.10.0.html)
@mroeschke, could you please take a look at this PR? Thanks @natmokval 