Improve --valid_states example

[colinbrislawn]

I was looking for a better example of filter_samples_from_otu_table.py, and found a great one inside the parse function. https://github.com/biocore/qiime/blob/master/qiime/parse.py#L695

This PR makes this example visible on the website.

[ghost]

Build results will soon be (or already are) available at: http://ci.qiime.org/job/qiime-github-pr/1724/