Add precision to planner.xy_skew_factor so it doesn't print as 0.00

Marlin/src/gcode/calibrate/M852.cpp

 
   if (!ijk) {
     SERIAL_ECHO_START();
-    SERIAL_ECHOPAIR(MSG_SKEW_FACTOR " XY: ", planner.xy_skew_factor);
+    SERIAL_ECHO(MSG_SKEW_FACTOR " XY: ");
+    SERIAL_ECHO_F(planner.xy_skew_factor, 6);
+    SERIAL_EOL();
     #if ENABLED(SKEW_CORRECTION_FOR_Z)
       SERIAL_ECHOPAIR(" XZ: ", planner.xz_skew_factor);
       SERIAL_ECHOLNPAIR(" YZ: ", planner.yz_skew_factor);