samples: subsys: task_wdt: Minor corrections in config and doc

Use the actual minimum timeout used by the sample for task watchdog
channels (100 ms) as the configuration value for the fallback hardware
watchdog, to avoid unnecessary executions of the timer handler.

Add missing indentation in the documentation of the sample so that
the console output is rendered correctly.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>