examples/hwapi/soft_pwm: Use Signal on()/off() methods.

Just one sample is updated with  on()/off() for now, there should be
remaining sample(s) showing .value() use (but more can be converted later,
as long as 1 or so good samples of .value() remains).

examples/hwapi/soft_pwm.py

@@ -14,10 +14,10 @@ def pwm_cycle(led, duty, cycles):
     duty_off = 20 - duty
     for i in range(cycles):
         if duty:
-            led.value(1)
+            led.on()
             utime.sleep_ms(duty)
         if duty_off:
-            led.value(0)
+            led.off()
             utime.sleep_ms(duty_off)