docs(mcp23009e): clarify GPIO naming for binary counter output pins

In  binary_counter example docstring

Author: Charly-sketch

lib/mcp23009e/examples/binary_counter.py

@@ -6,7 +6,8 @@
 LEFT  -> reset
 RIGHT -> print current value
 
-The value is displayed on GP0..GP3 as a 4-bit binary number.
+The value is displayed on GPIO1..GPIO4 (expander pins 0..3)
+as a 4-bit binary number.
 """
 
 from time import sleep_ms