Correct the argument spec.

playbooks/library/ec2_tag_local

@@ -101,7 +101,7 @@ def main():
     argument_spec = ec2_argument_spec()
     argument_spec.update(dict(
             resource = dict(required=True),
-            tags = dict(),
+            tags = dict(required=False, type='list'),
             state = dict(default='present', choices=['present', 'absent', 'list']),
         )
     )