Update README.md

Wrong Adafruit product ID.
Should be product ID 1946 (Adafruit FONA - Mini Cellular GSM Breakout uFL Version)
Was product  ID 1469 (Adafruit CC3000 WiFi Breakout with Onboard Ceramic Antenna)
Merge pull request #6 from czincke/master

Wrong Adafruit product ID in README.md