mux: fix grammar, missing "is".

This makes it easier to parse.

Signed-off-by: Peter Rosin <peda@axentia.se>
Link: https://lore.kernel.org/r/75067db8-0264-c49c-daa8-d7a877932711@axentia.se


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>