qemu-options.hx: fix -chardev ringbuf typos

Clean up the documentation for -chardev ringbuf.  There is a stray
closing parenthesis and the comma is unnecessary.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

qemu-options.hx

@@ -2374,7 +2374,7 @@ console with the given dimensions.
 @item -chardev ringbuf ,id=@var{id} [,size=@var{size}]
 
 Create a ring buffer with fixed size @option{size}.
-@var{size} must be a power of two, and defaults to @code{64K}).
+@var{size} must be a power of two and defaults to @code{64K}.
 
 @item -chardev file ,id=@var{id} ,path=@var{path}