GUACAMOLE-889: Fix mismatching attribute names

This patch targets the following error message:

line 30 objectClass: AttributeType not found: "protocol"
diff --git a/extensions/guacamole-auth-ldap/schema/guacConfigGroup.schema b/extensions/guacamole-auth-ldap/schema/guacConfigGroup.schema
index eff1766..129a41b 100644
--- a/extensions/guacamole-auth-ldap/schema/guacConfigGroup.schema
+++ b/extensions/guacamole-auth-ldap/schema/guacConfigGroup.schema
@@ -26,6 +26,6 @@
 objectClass ( 1.3.6.1.4.1.38971.1.2.1 NAME 'guacConfigGroup'
     DESC 'Guacamole configuration group'
     SUP groupOfNames
-    MUST protocol
-    MAY parameter )
+    MUST guacConfigProtocol
+    MAY guacConfigParameter )