add sane.exit() to example

Currently if example is run without sane.exit(), I see the
following error when script is ran next time:
`python _sane.error: Invalid argument`
Adding in sane.exit() avoids this error from happening on next run

Author: padraigconnolly

example.py

@@ -89,3 +89,8 @@
 # Close the device
 #
 dev.close()
+
+#
+# Exiting sane
+#
+sane.exit()