Fix for the commit dc2cb84 (Revert acceleration limiting to avoid overflow)

・Add forgotten '\'

Marlin/planner.cpp

   }
   else {
     #define LIMIT_ACCEL(AXIS) do{ \
-      if (max_acceleration_steps_per_s2[AXIS] < (accel * block->steps[AXIS]) / block->step_event_count)
-        accel = (max_acceleration_steps_per_s2[AXIS] * block->step_event_count) / block->steps[AXIS];
+      if (max_acceleration_steps_per_s2[AXIS] < (accel * block->steps[AXIS]) / block->step_event_count) \
+        accel = (max_acceleration_steps_per_s2[AXIS] * block->step_event_count) / block->steps[AXIS]; \
     }while(0)
 
     // Start with print or travel acceleration