add beer and coke to grammar for restaurant

ps-grammar/restaurant/restaurant.jsgf

@@ -9,7 +9,7 @@ public <restaurant> = [tobi] [please] <command> [please] [tobi];
 <objectCategory> = drinks | food;
 <objects> = <drinks> | <food>;
 
-<drinks> = milk;
+<drinks> = milk | coke | beer;
 <food> = pringles red | pringles green | pringles blue | cookies;
 
 <ordering> = ordering;