DOC/CLN: fixed bloolean indexing example, cleaned up typos
The boolean indexing example http://pandas.pydata.org/pandas-docs/version/0.16.0/indexing.html#boolean-indexing seems to be broken for a while. The variable `s` is being set in a much earlier part of the page and is not suitable for this example.

Also cleaned up a few minor typos. 

good catch, thanks!
