fix error

ps-grammar/gprs/gprs.jsgf

@@ -31,7 +31,7 @@ public <task> = [<robot_name>] [(would | could) you] [please] <command> [please]
 <grab_object> = <cleaning_class> | <drink_class> | <snack_class> | <food_class> | tool | (bathroom stuff) | cleaning_stuff | drinks | snacks | object | food | something | stuff | item | beverage | things;
 <cleaning_class> = brush | detergent | shampoo | sponge | toothpaste;
 <drink_class> = (orange juice) | beer | cola | chocolate_milk | energy_drink | grape_juice | milk | water;
-<snack_class> = chocolate_cookies | chips | biscuits | bread | pringles | strawberry_cookies;
+<snack_class> = chocolate_cookies | chips | biscuits | bread | pringles | (strawberry cookies);
 <food_class> = baby_food | crackers | coffee | yeast | noodles | beans; 
 
 <location> = <appliance_class> | <shelf_class> | <seat_class> | <table_class> | <door_class> | <exit_class> | exit | door | seating | utensil | location | room | appliance | shelf | seat | table | somewhere | apartment | furniture;