rxrpc: Fix the MAINTAINERS record

Fix the MAINTAINERS record so that it's more obvious who the maintainer for
AF_RXRPC is.

Reported-by: Joe Perches <joe@perches.com>
Reported-by: David Miller <davem@davemloft.net>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>