dt-bindings: iio: io-channel-mux: Fix compatible string in example code

The correct compatible string is "gpio-mux" (see
bindings/mux/gpio-mux.txt).

Cc: stable@vger.kernel.org # v4.13+
Reviewed-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Christian Eggers <ceggers@arri.de>
Link: https://lore.kernel.org/r/20200727101605.24384-1-ceggers@arri.de
Signed-off-by: Rob Herring <robh@kernel.org>