Improve HOTEND OFFSET Z display precision to 3 (#11685)

Marlin/src/gcode/config/M218.cpp

       SERIAL_CHAR(',');
       SERIAL_ECHO(hotend_offset[Y_AXIS][e]);
       SERIAL_CHAR(',');
-      SERIAL_ECHO(hotend_offset[Z_AXIS][e]);
+      SERIAL_ECHO_F(hotend_offset[Z_AXIS][e], 3);
     }
     SERIAL_EOL();
   }