docs(CONTRIBUTING.md): fixing link to Google Closure I18N library

The old seems to link to the source code of I18N. Found the same folder on their new Github repo.

"Closure Library's source repository has moved to GitHub.", https://code.google.com/p/closure-library/

Closes angular#7638

Author: HNygard

CONTRIBUTING.md

@@ -253,7 +253,7 @@ You can find out more detailed information about contributing in the
 
 
 
-[Google Closure I18N library]: https://code.google.com/p/closure-library/source/browse/closure/goog/i18n/
+[Google Closure I18N library]: https://github.com/google/closure-library/tree/master/closure/goog/i18n
 [angular-dev]: https://groups.google.com/forum/?fromgroups#!forum/angular-dev
 [coc]: https://github.com/angular/code-of-conduct/blob/master/CODE_OF_CONDUCT.md
 [commit-message-format]: https://docs.google.com/document/d/1QrDFcIiPjSLDn3EL15IJygNPiHORgU1_OOAqWjiDU5Y/edit#