Remove `memberOf` OID in LDAP `user_filter` docs

While not technically invalid, it is not necessary to have the 
`memberOf` OID in the `user_filter`. It clutters things up and causes
confusion for users so it's better if we remove it from the docs. 

doc/administration/auth/ldap.md

@@ -221,7 +221,7 @@ Tip: If you want to limit access to the nested members of an Active Directory
 group you can use the following syntax:
 
 ```
-(memberOf:1.2.840.113556.1.4.1941:=CN=My Group,DC=Example,DC=com)
+(memberOf=CN=My Group,DC=Example,DC=com)
 ```
 
 Please note that GitLab does not support the custom filter syntax used by