Fix documentation for Github integration authorization callback url.

There are two callbacks that could be used with Github integration:

  * /import/github/callback (used by project import)
  * /users/auth/github/callback (used by OmniAuth)

Github's documentation suggests that authorization callback url should be
set to the longest common path.

https://developer.github.com/v3/oauth/#redirect-urls

Configuring according to the previous documentation resulted in a
redirect_uri_mismatch error from Github when logging in via OmniAuth.

doc/integration/github.md

@@ -19,7 +19,7 @@ GitHub will generate an application ID and secret key for you to use.
     - Application name: This can be anything. Consider something like "\<Organization\>'s GitLab" or "\<Your Name\>'s GitLab" or something else descriptive.
     - Homepage URL: The URL to your GitLab installation. 'https://gitlab.company.com'
     - Application description: Fill this in if you wish.
-    - Default authorization callback URL is '${YOUR_DOMAIN}/import/github/callback'
+    - Authorization callback URL is 'http(s)://${YOUR_DOMAIN}'
 1.  Select "Register application".
 
 1.  You should now see a Client ID and Client Secret near the top right of the page (see screenshot).