fix 2 broken links

* bad copy-paste: our main page links *BUILD files* page with another page's title (*Conceptual Overview*). so use correct title
* python_requirement_library should link to python_requirement, but link is misplaced and broken. so fix and fix

Reviewed at https://rbcommons.com/s/twitter/r/1550/
Sync of https://github.com/pantsbuild/pants/commit/d539bc81a04c06a01a4897005602da0f54d387a1