Clean up the documentation for -chardev ringbuf.  There is a stray
closing parenthesis and the comma is unnecessary.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>