Fix omniauth link, site has moved.

doc/integration/omniauth.md

@@ -168,7 +168,7 @@ want their accounts to be upgraded to full internal accounts.
 >**Note:**
 The following information only applies for installations from source.
-GitLab uses [Omniauth](http://www.omniauth.org/) for authentication and already ships
+GitLab uses [Omniauth](https://github.com/omniauth/omniauth) for authentication and already ships
 with a few providers pre-installed (e.g. LDAP, GitHub, Twitter). But sometimes that
 is not enough and you need to integrate with other authentication solutions. For
 these cases you can use the Omniauth provider.