Fix up example to use correct object

Currently the example fails with: python3 pydexarm/example.py
pydexarm: /dev/ttyACM0 open
Traceback (most recent call last):
  File "/home/michele/Devel/DexArm_API/pydexarm/example.py", line 9, in <module>
    dexarm.go_home()
NameError: name 'dexarm' is not defined

Let's point it to the proper class so things start working again.
Renaming 'dexarm' instead of 'device' in order to make it less
confusing.

Signed-off-by: Michele Baldessari <michele@acksyn.org>

Author: mbaldessari

pydexarm/example.py

@@ -5,15 +5,15 @@
 '''mac & linux'''
 # device = Dexarm(port="/dev/tty.usbmodem3086337A34381")
 
-dexarm.go_home()
+device.go_home()
 
-dexarm.move_to(50, 300, 0)
-dexarm.move_to(50, 300, -50)
-dexarm.air_picker_pick()
-dexarm.move_to(50, 300, 0)
-dexarm.move_to(-50, 300, 0)
-dexarm.move_to(-50, 300, -50)
-dexarm.air_picker_place()
+device.move_to(50, 300, 0)
+device.move_to(50, 300, -50)
+device.air_picker_pick()
+device.move_to(50, 300, 0)
+device.move_to(-50, 300, 0)
+device.move_to(-50, 300, -50)
+device.air_picker_place()
 
-dexarm.go_home()
-dexarm.close()
+device.go_home()
+device.close()