beagley-ai-using-pwm: use dev/hat/pwm symlink, pwmchipX can change

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>

boards/beagley/ai/demos/beagley-ai-using-pwm.rst

@@ -43,7 +43,7 @@ In the editor copy paste the script content below,
 
     #!/bin/bash
 
-    PWMPIN="/sys/devices/platform/bus@f0000/23000000.pwm/pwm/pwmchip3/pwm1"
+    PWMPIN="/dev/hat/pwm/hat-08-pwm"
 
 
     echo 1000 > $PWMPIN/period
@@ -92,4 +92,4 @@ Troubleshooting
 Going Further
 *******************
 
-.. todo:: Fill out empty section
+.. todo:: Fill out empty section
\ No newline at end of file