Fix typo.

Co-authored-by: Travis Ralston <travisr@matrix.org>

proposals/3083-restricted-rooms.md

@@ -55,7 +55,7 @@ not a member of at least one of the rooms, the homeserver should return an error
 response with HTTP status code of 403 and an `errcode` of `M_FORBIDDEN`.
 
 It is possible for a homeserver receiving a `/make_join` / `/send_join` request
-to not know if the user is in some of the allowed room (due to not participating
+to not know if the user is in some of the allowed rooms (due to not participating
 in them). Any allow room that the homeserver cannot verify the membership should
 be treated as if the user is not in that room. If the user is not in any of the
 rooms (or some of the rooms cannot be verified) the homeserver should reject the