Make it so that index.php displays something useful if php not installed

If php is not installed, and index.php is treated as an html file,
it will just display "run()", since the rest is eaten up by the
<?php pseudo-tag/processing instruction. Add a comment to close that
and open a <pre> tag, so users can see the little help text telling
them they need to install php

Change-Id: Ib23fc9b1464309b7ef31f1c22b5a96bd3d6d2611

index.php

@@ -1,5 +1,7 @@
 <?php
 /**
+ * For people without PHP, close tag for easier debugging: ><pre>
+ *
  * This is the main web entry point for MediaWiki.
  *
  * If you are reading this in your web browser, your server is probably