bigint: fix and un-ignore the first doctest
bigint: fix and un-ignore the first doctest

koverstreet/num,rust-num/num,rust-num/num-bigint,DonSheddow/num,vks/num,vks/num,rust-lang/num,rust-num/num-bigint,rust-num/num,DonSheddow/num,rust-num/num-traits,rust-num/num-integer,rust-num/num-integer,rust-lang/num,koverstreet/num,rust-num/num-traits