samples: threads: cleanups

Align the docs and source code with other changes done to the basic
samples.

Use numbers 0 and 1 to refer to the LEDs consistently. This matches
the generic devicetree aliases used by Zephyr, instead of using USR1
and USR2, which are specific to 96b_carbon.

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