Mixed up LED pin definitions for Maix-BIT

[ManDeJan]

https://github.com/sipeed/Maixduino/blob/master/variants/sipeed_maix_bit/pins_arduino.h

This files defines led pins as follows: Green -> 13 Blue -> 12 Red -> 14

Testing this on my board and the according to the Sipeed Maix-BIT Specifications v2.0, the defines should be as follows: Green -> 12 Red -> 13 Blue -> 14