Fixes #3313 : Wrong board def in pins.h

Marlin/pins.h

@@ -151,7 +151,7 @@
   #include "pins_AJ4P.h"
 #elif MB(MKS_13)
   #include "pins_MKS_13.h"
-#elif MB(BOARD_SAINSMART_2IN1)
+#elif MB(SAINSMART_2IN1)
   #include "pins_SAINSMART_2IN1.h"
 #else
   #error Unknown MOTHERBOARD value set in Configuration.h