Use 4.0 method of connecting to and emitting signals (#6311)

Change docs to use the new method of connecting and emitting signals

getting_started/first_2d_game/03.coding_the_player.rst

@@ -478,7 +478,7 @@ this code to the function:
 
     func _on_Player_body_entered(body):
         hide() # Player disappears after being hit.
-        emit_signal("hit")
+        hit.emit()
         # Must be deferred as we can't change physics properties on a physics callback.
         $CollisionShape2D.set_deferred("disabled", true)