Update README.md

In example part,   
vapid_claims={
                "sub": "YourNameHere@example.org",
            }
It will lead to an error which lacks "mailto" fields.

README.md

@@ -98,7 +98,7 @@ try:
         data="Mary had a little lamb, with a nice mint jelly",
         vapid_private_key="path/to/vapid_private.pem",
         vapid_claims={
-                "sub": "YourNameHere@example.org",
+                "sub": "mailto:YourNameHere@example.org",
             }
     )
 except WebPushException as ex: