Fix comment in watchdog.

Fix comment in watchdog.

Fix a comment in watchdog_state_machine_standby(). When commit b2f1526958 changed from:
last_rcv_sec >= (2 * BEACON_MESSAGE_INTERVAL_SECONDS))
to:
last_rcv_sec >= (3 * BEACON_MESSAGE_INTERVAL_SECONDS))
The comment below was left unchanged "two" instead of "three".
Also fix small typo.

Backpatch-though: master only