Fix inconsistent switch case syntax

This must have been a typo, since it's the only case in the project using the non-standard syntax.

Author: theodorejb

library/Zend/Pdf/Resource/Image/Png.php

@@ -190,7 +190,7 @@ public function __construct($imageFileName)
                     fseek($imageFile, 4, SEEK_CUR); //4 Byte Ending Sequence
                     break;
 
-                case 'IEND';
+                case 'IEND':
                     break 2; //End the loop too
 
                 default: