Fix broken link in Code Base documentation.
Fix broken link in Code Base documentation.

On *Running Unit Tests* page update hyperlink syntax to correctly reference
nose usage guide.

Testing Done:
Built docs and verified that link now goes to the desired page rather than
producing a 404.

Reviewed at https://reviews.reviewboard.org/r/8251/

davidt/reviewboard,sgallagher/reviewboard,davidt/reviewboard,sgallagher/reviewboard,davidt/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,brennie/reviewboard,chipx86/reviewboard,chipx86/reviewboard,sgallagher/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,brennie/reviewboard,brennie/reviewboard,davidt/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,brennie/reviewboard,chipx86/reviewboard