Fix capitalization of counter_m

Marlin/stepper.cpp

 volatile signed char Stepper::count_direction[NUM_AXIS] = { 1, 1, 1, 1 };
 
 #if ENABLED(MIXING_EXTRUDER)
-  long Stepper::counter_M[MIXING_STEPPERS];
+  long Stepper::counter_m[MIXING_STEPPERS];
 #endif
 
 unsigned short Stepper::acc_step_rate; // needed for deceleration start point
 
       #if ENABLED(MIXING_EXTRUDER)
         MIXING_STEPPERS_LOOP(i)
-          counter_M[i] = -(current_block->mix_event_count[i] >> 1);
+          counter_m[i] = -(current_block->mix_event_count[i] >> 1);
       #endif
 
       step_events_completed = 0;
           // Tick the counters used for this mix
           MIXING_STEPPERS_LOOP(j) {
             // Step mixing steppers (proportionally)
-            counter_M[j] += current_block->steps[E_AXIS];
+            counter_m[j] += current_block->steps[E_AXIS];
             // Step when the counter goes over zero
-            if (counter_M[j] > 0) En_STEP_WRITE(j, !INVERT_E_STEP_PIN);
+            if (counter_m[j] > 0) En_STEP_WRITE(j, !INVERT_E_STEP_PIN);
           }
         #else // !MIXING_EXTRUDER
           PULSE_START(E);
             count_position[E_AXIS] += count_direction[E_AXIS];
           }
           MIXING_STEPPERS_LOOP(j) {
-            if (counter_M[j] > 0) {
-              counter_M[j] -= current_block->mix_event_count[j];
+            if (counter_m[j] > 0) {
+              counter_m[j] -= current_block->mix_event_count[j];
               En_STEP_WRITE(j, INVERT_E_STEP_PIN);
             }
           }

Marlin/stepper.h

     // Mixing extruder mix counters
     //
     #if ENABLED(MIXING_EXTRUDER)
-      static long counter_M[MIXING_STEPPERS];
+      static long counter_m[MIXING_STEPPERS];
       #define MIXING_STEPPERS_LOOP(VAR) \
         for (uint8_t VAR = 0; VAR < MIXING_STEPPERS; VAR++) \
           if (current_block->mix_event_count[VAR])