Docs: update GFX Pack readme

micropython/modules_py/gfx_pack.md

@@ -71,6 +71,7 @@ example:
 
 ```python
 display = board.display
+display.set_pen(15)
 display.text("Hello World!", 0, 0)
 display.line(0, 0, 128, 64) 
 display.update()  # Update display with the above items
@@ -88,4 +89,4 @@ display = board.display
 display.set_backlight(0.0)  # Backlight is off
 display.set_backlight(0.5)  # Backlight is 50%
 display.set_backlight(1.0)  # Backlight is 100%
-```
+```