Minor edit to coupon example to correct an error.

I was having trouble getting this to work:

    coupon = Chargify::Subscription::Coupon.find_by_product_family_id_and_code(2,"10OFF")

as it's shown in the coupon example on line 25.

I noticed all the other examples use this syntax instead: 

    coupon = Chargify::Coupon.find_by_product_family_id_and_code(2,"10OFF")

which works perfect.

I'm not sure if this is just an issue with me using it wrong, but it seems like a minor error.