example: "all of the"

thanks @sej7278

Author: dcousens

examples/MakefileExample/Makefile-example.mk

@@ -1,6 +1,6 @@
 ### DISCLAIMER
 ### This is an example Makefile and it MUST be configured to suit your needs.
-### For detailed explanations about all the available options, please refer
+### For detailed explanations about all of the available options, please refer
 ### to https://github.com/sudar/Arduino-Makefile/blob/master/arduino-mk-vars.md
 ### Original project where this Makefile comes from: https://github.com/WeAreLeka/Bare-Arduino-Project