Fix calculation of BAL grid fitting into the bed

The previous calc was wrong. Between N points there are only N-1 gaps,
not N.

So changing AUTO_BED_LEVELING_GRID_POINTS to
(AUTO_BED_LEVELING_GRID_POINTS-1)

Marlin/Configuration.h

 
   #ifdef AUTO_BED_LEVELING_GRID	// Check if Probe_Offset * Grid Points is greater than Probing Range
     #if X_PROBE_OFFSET_FROM_EXTRUDER < 0
-      #if (-(X_PROBE_OFFSET_FROM_EXTRUDER * AUTO_BED_LEVELING_GRID_POINTS) >= (RIGHT_PROBE_BED_POSITION - LEFT_PROBE_BED_POSITION))
+      #if (-(X_PROBE_OFFSET_FROM_EXTRUDER * (AUTO_BED_LEVELING_GRID_POINTS-1)) >= (RIGHT_PROBE_BED_POSITION - LEFT_PROBE_BED_POSITION))
 	     #error "The X axis probing range is not enough to fit all the points defined in AUTO_BED_LEVELING_GRID_POINTS"
 	  #endif
 	#else
-      #if ((X_PROBE_OFFSET_FROM_EXTRUDER * AUTO_BED_LEVELING_GRID_POINTS) >= (RIGHT_PROBE_BED_POSITION - LEFT_PROBE_BED_POSITION))
+      #if ((X_PROBE_OFFSET_FROM_EXTRUDER * (AUTO_BED_LEVELING_GRID_POINTS-1)) >= (RIGHT_PROBE_BED_POSITION - LEFT_PROBE_BED_POSITION))
 	     #error "The X axis probing range is not enough to fit all the points defined in AUTO_BED_LEVELING_GRID_POINTS"
 	  #endif
 	#endif
     #if Y_PROBE_OFFSET_FROM_EXTRUDER < 0
-      #if (-(Y_PROBE_OFFSET_FROM_EXTRUDER * AUTO_BED_LEVELING_GRID_POINTS) >= (BACK_PROBE_BED_POSITION - FRONT_PROBE_BED_POSITION))
+      #if (-(Y_PROBE_OFFSET_FROM_EXTRUDER * (AUTO_BED_LEVELING_GRID_POINTS-1)) >= (BACK_PROBE_BED_POSITION - FRONT_PROBE_BED_POSITION))
 	     #error "The Y axis probing range is not enough to fit all the points defined in AUTO_BED_LEVELING_GRID_POINTS"
 	  #endif
 	#else
-      #if ((Y_PROBE_OFFSET_FROM_EXTRUDER * AUTO_BED_LEVELING_GRID_POINTS) >= (BACK_PROBE_BED_POSITION - FRONT_PROBE_BED_POSITION))
+      #if ((Y_PROBE_OFFSET_FROM_EXTRUDER * (AUTO_BED_LEVELING_GRID_POINTS-1)) >= (BACK_PROBE_BED_POSITION - FRONT_PROBE_BED_POSITION))
 	     #error "The Y axis probing range is not enough to fit all the points defined in AUTO_BED_LEVELING_GRID_POINTS"
 	  #endif
 	#endif