Fix return type of addStorageWrapper in PHPDoc

lib/public/files/storage/istoragefactory.php

@@ -19,7 +19,7 @@ interface IStorageFactory {
 	 *
 	 * @param string $wrapperName
 	 * @param callable $callback
-	 * @return true if the wrapper was added, false if there was already a wrapper with this
+	 * @return bool true if the wrapper was added, false if there was already a wrapper with this
 	 * name registered
 	 */
 	public function addStorageWrapper($wrapperName, $callback);