Fix(usage): don't print defaults of required positionals

[ulidtko]

Hi! Nice library :)

Sending you a oneliner patch, hopefully trivial enough to review & merge in 2 clicks.

See, if I'm doing a (non-optional!) positional argument, the help message comes out a little awkward, [required, default: 0]:

usage: foobar [-S UINT64] n

options:
    -S, --seed UINT64        RNG seed to start from [default: 8214974914043046976]

positionals:
    n                        permutation length [required, default: 0]

It kind of makes no sense to talk about defaults of args the user can't omit.