Merge pull request #1788 from AnHardt/iss1786

Fix typo in Marlin_main.cpp

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