Add missing "omniauth" prefix to option in docs [ci skip]

Changes block_auto_created_users to omniauth_block_auto_created_users,
otherwise the option is ignored. Fixes #3319.

CHANGELOG

@@ -17,6 +17,7 @@ v 8.2.0 (unreleased)
   - Remove deprecated CI events from project settings page
   - Use issue editor as cross reference comment author when issue is edited with a new mention.
   - [API] Add ability to fetch the commit ID of the last commit that actually touched a file
+  - Fix omniauth documentation setting for omnibus configuration (Jon Cairns)
 
 v 8.1.1
   - Fix cloning Wiki repositories via HTTP (Stan Hu)

doc/integration/omniauth.md

@@ -36,7 +36,7 @@ If you want to change these settings:
     ```
     gitlab_rails['omniauth_enabled'] = true
     gitlab_rails['omniauth_allow_single_sign_on'] = false
-    gitlab_rails['block_auto_created_users'] = true
+    gitlab_rails['omniauth_block_auto_created_users'] = true
     ```
 
 * **For installations from source**