games/doom: fix unterminated comment causing sound bugs (from qu7uux)

this bug was introduced in the plan9 port, and since i_sound.c compiles with
no warning, it was never noticed. in effect, the statement between the
unterminated comment and the next is ignored. channelids[] is used in addsfx()
to avoid re-adding certain sounds if they are already playing. one of those is
sfx_sawful, and because of this bug, it is added again each tic during which
the player fires the chainsaw, rather than reset every tic.
compare firing the chainsaw continuously with and without the patch (without
hitting an enemy).

sys/src/games/doom/i_sound.c

@@ -430,6 +430,7 @@ addsfx(int id, int vol, int step, int sep)
 
 	/* Preserve sound SFX id,
 	**  e.g. for avoiding duplicates of chainsaw.
+	*/
 	channelids[slot] = id;
 
 	/* You tell me. */