game_api.txt: Add new metal sounds func.

game_api.txt

@@ -464,6 +464,7 @@ Sounds inside the default table can be used within the sounds field of node defi
  * `default.node_sound_wood_defaults()`
  * `default.node_sound_leaves_defaults()`
  * `default.node_sound_glass_defaults()`
+ * `default.node_sound_metal_defaults()`
 
 Default constants
 -----------------