Use moves_free in ok_to_send

Marlin/src/gcode/queue.cpp

@@ -253,7 +253,7 @@ void GCodeQueue::ok_to_send() {
       while (NUMERIC_SIGNED(*p))
         SERIAL_ECHO(*p++);
     }
-    SERIAL_ECHOPAIR_P(SP_P_STR, int(BLOCK_BUFFER_SIZE - planner.movesplanned() - 1));
+    SERIAL_ECHOPAIR_P(SP_P_STR, int(planner.moves_free()));
     SERIAL_ECHOPAIR(" B", int(BUFSIZE - length));
   #endif
   SERIAL_EOL();