[ticket/16449] Rename template variable PRE to S_PRE_STEP

PHPBB3-16449

phpBB/adm/style/acp_ext_disable.html

@@ -7,7 +7,7 @@
 	<p>{{ lang('EXTENSIONS_EXPLAIN') }}</p>
 	<p>{{ lang('EXTENSION_DISABLE_EXPLAIN') }}</p>
 
-	{% if PRE %}
+	{% if S_PRE_STEP %}
 		<fieldset>
 			<h2>{{ lang('CONFIRM') }}</h2>
 			<p>{{ CONFIRM_MESSAGE }}</p>