Correct MOTHERBOARD for Geeetech i3 Pro X

Marlin/src/config/examples/Geeetech/I3_Pro_X-GT2560/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_ULTIMAKER
+  #define MOTHERBOARD BOARD_GT2560_REV_A_PLUS
 #endif
 
 // Optional custom name for your RepStrap or other custom machine