samples: blinky_pwm: fix missing project renaming

The commit bfb10406 has renamed the
sample blink_led to blinky_pwm, however, the project still have the
former name: blink_led.

This renames the project to its new name: blinky_pwm.

Signed-off-by: Gaël PORTAY <gael.portay@gmail.com>