samples: mesh: clean up

Drop device_get_binding() for microbit, using a gpio_dt_spec instead.
Rename the 'button' global to 'button_work' since it's a k_work
related to the button and does not actually represent the button
itself.

Fix a typo in the docs.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>