Fix the m.room.member.yaml example

This is a `join` event, and the `sender` doesn't match the `state_key`, so the event couldn't pass the authorization rules.

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

data/event-schemas/examples/m.room.member.yaml

@@ -1,6 +1,7 @@
 {
   "$ref": "core/state_event.json",
   "state_key": "@alice:example.org",
+  "sender": "@alice:example.org",
   "type": "m.room.member",
   "content": {
     "membership": "join",