Really validate that $wgArticlePath starts with a slash

The regular expression wasn't entirely correct.
Follow-up to a4a3d04.

Bug: T48998
Change-Id: I08bdf2db20c1c3de55527fc812bcbb55fa23f7bc

includes/Setup.php

@@ -514,12 +514,12 @@
 $ps_validation = Profiler::instance()->scopedProfileIn( $fname . '-validation' );
 
 // T48998: Bail out early if $wgArticlePath is non-absolute
-if ( !preg_match( '/^https?:\/\/|\//', $wgArticlePath ) ) {
+if ( !preg_match( '/^(https?:\/\/|\/)/', $wgArticlePath ) ) {
 	throw new FatalError(
-		'If you use a relative URL on $wgArticlePath, it must start ' .
-		'with a slash (/).<br><br>See ' .
+		'If you use a relative URL for $wgArticlePath, it must start ' .
+		'with a slash (<code>/</code>).<br><br>See ' .
 		'<a href="https://www.mediawiki.org/wiki/Manual:$wgArticlePath">' .
-		'https://www.mediawiki.org/wiki/Manual:$wgArticlePath</a>'
+		'https://www.mediawiki.org/wiki/Manual:$wgArticlePath</a>.'
 	);
 }