Adds steps for disabling 2fa for single user

doc/administration/troubleshooting/gitlab_rails_cheat_sheet.md

@@ -557,10 +557,17 @@ end
 ### Skip reconfirmation
 
 ```ruby
-user = User.find_by_username '<username>'
+user = User.find_by_username('<username>')
 user.skip_reconfirmation!
 ```
 
+### Disable 2fa for single user
+
+```ruby
+user = User.find_by_username('<username>')
+user.disable_two_factor!
+```
+
 ### Active users & Historical users
 
 ```ruby