Widen parameter type

This exception is used in two places, one where $generatedMode is
clearly a string, and another where typing it as a string will cause a
type error, because $generatedMode is supposed to be an int there.

lib/Doctrine/ORM/Mapping/MappingException.php

@@ -824,7 +824,8 @@ public static function invalidFetchMode($className, $annotation)
         return new self("Entity '" . $className . "' has a mapping with invalid fetch mode '" . $annotation . "'");
     }
 
-    public static function invalidGeneratedMode(string $annotation): MappingException
+    /** @param int|string $annotation */
+    public static function invalidGeneratedMode($annotation): MappingException
     {
         return new self("Invalid generated mode '" . $annotation . "'");
     }