Fix Google API callback

https://gitlab.com/gitlab-org/gitlab-ce/issues/38911#note_42966393

doc/integration/google.md

@@ -30,7 +30,7 @@ In Google's side:
 
         ```
         https://gitlab.example.com/users/auth/google_oauth2/callback
-        https://gitlab.example.com/google_api/auth/callback
+        https://gitlab.exampl.com/-/google_api/auth/callback
         ```
 
 1. You should now be able to see a Client ID and Client secret. Note them down