Fix example file to pull in correct pins file

Marlin/src/config/examples/Formbot/T-Rex_2+/Configuration.h

 // The following define selects which electronics board you have.
 // Please choose the name from boards.h that matches your setup
 #ifndef MOTHERBOARD
-//#define MOTHERBOARD BOARD_RUMBA
-#define MOTHERBOARD BOARD_FORMBOT
+  #define MOTHERBOARD BOARD_FORMBOT_TREX2
 #endif
 
 // Optional custom name for your RepStrap or other custom machine