Fix typo in Marlin_main.cpp

related to current_position[Z_AXIS] and retract_zlift.

'+ =' -> '+='

Fix for #1786

Marlin/Marlin_main.cpp

     else {
 
       if (retract_zlift > 0.01) {
-        current_position[Z_AXIS] + =retract_zlift;
+        current_position[Z_AXIS] += retract_zlift;
         #ifdef DELTA
           sync_plan_position_delta();
         #else