The instructions for that brew are a little skimpy. It says "steep for 20 minutes or until water reaches 170 degrees". The intent is to throw the grain bag in cold water, turn on the heat, and wait until it reaches 170, not to steep for 20 minutes at 170. Too long at 170 could potentially release some tannins, but I wouldn't sweat it. You could have thrown the grain bag in at 170, shut off the heat, and let it steep for 20 minutes. Steeping is not a super-critical thing, so you'll be OK.