Fix typo

Author: iMattPro

language/en/common.php

@@ -43,7 +43,7 @@
 	'IDEAS_TITLE'			=> 'phpBB Ideas',
 	'IDEAS_NOT_AVAILABLE'	=> 'Ideas is not available at this time.',
 	'IMPLEMENTED'           => 'Implemented',
-	'IMPLEMENTED_ERROR'		=> 'Mst be a valid phpBB version number.',
+	'IMPLEMENTED_ERROR'		=> 'Must be a valid phpBB version number.',
 	'IMPLEMENTED_IDEAS'		=> 'Recently Implemented Ideas',
 	'IMPLEMENTED_VERSION'	=> 'phpBB version',
 	'IN_PROGRESS'           => 'In Progress',