Fixed typo in arc

Marlin/motion_control.cpp

   float millimeters_of_travel = hypot(angular_travel*radius, fabs(linear_travel));
   if (millimeters_of_travel < 0.001) { return; }
   uint16_t segments = floor(millimeters_of_travel/MM_PER_ARC_SEGMENT);
-  if(segments = 0) segments = 1;
+  if(segments == 0) segments = 1;
   
   /*  
     // Multiply inverse feed_rate to compensate for the fact that this movement is approximated