Disable allow_username_or_email_login in example

The example LDAP configuration in gitlab.yml enables the
allow_username_or_email_login setting. Because the effect of this
setting is somewhat counterintuitive, I propose we make 'false' the
example default. The settings initializer already sets this setting to
'false'.

config/gitlab.yml.example

@@ -143,7 +143,7 @@ production: &base
     #
     # If you are using "uid: 'userPrincipalName'" on ActiveDirectory you need to
     # disable this setting, because the userPrincipalName contains an '@'.
-    allow_username_or_email_login: true
+    allow_username_or_email_login: false
 
     # Base where we can search for users
     #