rp2/README: Remove redundant global statement from example code.

Since `led` is not being rebound inside the `tick` function the standard
Python name resolution method will find it.

Signed-off-by: Steve Holden <[email protected]>

Author: holdenweb

ports/rp2/README.md

@@ -69,7 +69,6 @@ from machine import Pin, Timer
 led = Pin(25, Pin.OUT)
 tim = Timer()
 def tick(timer):
-    global led
     led.toggle()
 
 tim.init(freq=2.5, mode=Timer.PERIODIC, callback=tick)