doc: remove incorrect grammar for ALTER EVENT TRIGGER

The Parameters subsection had an extra TRIGGER in the grammar
for DISABLE/ENABLE which is incorrect.  Backpatch down to all
supported versions since it's been like this all along.

Discussion: https://postgr.es/m/0AFB171E-7E78-4A90-A140-46AB270212CA@yesql.se
Backpatch-through: v12

Author: danielgustafsson

doc/src/sgml/ref/alter_event_trigger.sgml

@@ -73,7 +73,7 @@ ALTER EVENT TRIGGER <replaceable class="parameter">name</replaceable> RENAME TO
    </varlistentry>
 
    <varlistentry>
-    <term><literal>DISABLE</literal>/<literal>ENABLE [ REPLICA | ALWAYS ] TRIGGER</literal></term>
+    <term><literal>DISABLE</literal>/<literal>ENABLE [ REPLICA | ALWAYS ]</literal></term>
     <listitem>
      <para>
       These forms configure the firing of event triggers.  A disabled trigger