Fix error message for probe region sanity check

Marlin/SanityCheck.h

         #if LEFT_PROBE_BED_POSITION > RIGHT_PROBE_BED_POSITION
           #error LEFT_PROBE_BED_POSITION must be less than RIGHT_PROBE_BED_POSITION.
         #elif FRONT_PROBE_BED_POSITION > BACK_PROBE_BED_POSITION
-          #error BACK_PROBE_BED_POSITION must be less than FRONT_PROBE_BED_POSITION.
+          #error FRONT_PROBE_BED_POSITION must be less than BACK_PROBE_BED_POSITION.
         #endif
         // Make sure probing points are reachable
         #if LEFT_PROBE_BED_POSITION < MIN_PROBE_X