Doc fixes: Minor typo (was fixed on 0.16 branch so I figured it could be on master as well) and an error in the overview doc.

The first XPath example does in fact get the description of the torrent. The second, however, gets the file size (which is why it's talking about the file size directly above it). Corrected, so new users can better understand how to use scrapy and XPath without erroneous information