Change --nth option for CTRL-R key binding (#90)

Remove `1` from --nth option. With the change you can no more use `$`
anchor to match the tail of a command index. But it makes search
around 15% faster.

    jg@jg:~> time cat history | fzf +s -n..,1,2.. -f fzf > /dev/nul
    real    0m2.929s
    user    0m2.766s
    sys     0m0.154s

    jg@jg:~> time cat history | fzf +s -n2..,.. -f fzf > /dev/null
    real    0m2.535s
    user    0m2.422s
    sys     0m0.112s