Add note about removing new_game() _ready() in 2D tut (#6781)

* Add note about removing new_game() _ready() in 2D tut

The Start button handles calling `new_game()` so without removing the `_ready()` function that called `new_game()`, the game will automatically start on launch.

* Revise 2d tut new_game() removal to be a reminder

Based on feedback from cbscribe since it's mentioned in the
previous part of the tutorial.

getting_started/first_2d_game/06.heads_up_display.rst

@@ -387,6 +387,9 @@ In ``game_over()`` we need to call the corresponding ``HUD`` function:
 
         _hud->show_game_over();
 
+Just a reminder: we don't want to start the new game automatically, so
+remove the call to ``new_game()`` in ``_ready()`` if you haven't yet.
+
 Finally, add this to ``_on_ScoreTimer_timeout()`` to keep the display in sync
 with the changing score: