clarified parsing example for MEGA users

following the instructions in the comments while using a MEGA was wrong:
* "comment out the above six lines" - no six lines to comment out.
* mySerial was called on line 83 when requesting firmware version, and would
  not have been instantiated if using just "Adafruit_GPS GPS(&Serial1);"
Merge pull request #28 from tapiralec/master

clarified parsing example for MEGA users