Update help page link from TBPL to Treeherder (1033241)

This work fixes Bugzilla bug [1033241](https://bugzilla.mozilla.org/show_bug.cgi?id=1033241).

This corrects the help page link, pointing the Help page from the legacy TBPL Wiki to the new Treeherder Wiki.

Tested on Windows:
FF Release **31.0**
Chrome Latest Release **36.0.1985.125 m**

Adding @camd for visibility.