docs(CONTRIBUTING): fix broken link to GitHub PR Helper

Closes angular#5134

Author: dee-kap

CONTRIBUTING.md

@@ -133,7 +133,7 @@ We track Pull Requests by attaching labels and assigning to milestones.  For som
 does not provide a good UI for managing labels on Pull Requests (unlike Issues).  We have developed
 a simple Chrome Extension that enables you to view (and manage if you have permission) the labels
 on Pull Requests.  You can get the extension from the Chrome WebStore -
-[GitHub PR Helper](github-pr-helper)
+[GitHub PR Helper][github-pr-helper]
 
 ## Coding Rules
 To ensure consistency throughout the source code, keep these rules in mind as you are working:
@@ -243,4 +243,4 @@ You can find out more detailed information about contributing in the
 [individual-cla]: http://code.google.com/legal/individual-cla-v1.0.html
 [corporate-cla]: http://code.google.com/legal/corporate-cla-v1.0.html
 [commit-message-format]: https://docs.google.com/document/d/1QrDFcIiPjSLDn3EL15IJygNPiHORgU1_OOAqWjiDU5Y/edit#
-[github-pr-helper]: https://chrome.google.com/webstore/detail/github-pr-helper/mokbklfnaddkkbolfldepnkfmanfhpen
+[github-pr-helper]: https://chrome.google.com/webstore/detail/github-pr-helper/mokbklfnaddkkbolfldepnkfmanfhpen