Fix Z_HEAD index for COREYZ

Marlin/enum.h

   E_AXIS  = 3,
   X_HEAD  = 4,
   Y_HEAD  = 5,
-  Z_HEAD  = 5
+  Z_HEAD  = 6
 };
 
 #define LOOP_XYZ(VAR)  for (uint8_t VAR=X_AXIS; VAR<=Z_AXIS; VAR++)

Marlin/planner.cpp

    * Having the real displacement of the head, we can calculate the total movement length and apply the desired speed.
    */
   #if ENABLED(COREXY) || ENABLED(COREXZ) || ENABLED(COREYZ)
-    float delta_mm[6];
+    float delta_mm[7];
     #if ENABLED(COREXY)
       delta_mm[X_HEAD] = dx * steps_to_mm[A_AXIS];
       delta_mm[Y_HEAD] = dy * steps_to_mm[B_AXIS];