docs(readme): download the latest release in the install snippet

[haribo]

Summary

README.md:39 told a newcomer to gh release download v0.10.0 — three releases behind. The
paragraph is about SHA256SUMS being published from v0.10.0, so nothing was false; the
copyable line just installed an old binary, and would be wrong again after every release.

The fix the issue proposed does not work, which running it before writing it down is the
only reason this PR knows:

$ gh release download --repo haribo/shellf
`--pattern` or `--archive` is required when downloading the latest release

So the tagless form must name assets — and once it does, sha256sum -c SHA256SUMS fails on the
architecture that was not downloaded. --ignore-missing is required, and was checked not to
weaken the verification:

case	result
binary never downloaded	SHA256SUMS: no file was verified → exit 1
binary tampered with (one byte appended)	shellf-linux-amd64: FAILED → exit 1
downloaded and intact	shellf-linux-amd64: OK → exit 0

Test plan

The snippet was run as written, against the live release:

$ gh release download --repo haribo/shellf --pattern 'shellf-linux-amd64' --pattern 'SHA256SUMS'
$ sha256sum -c --ignore-missing SHA256SUMS
shellf-linux-amd64: OK
$ chmod +x shellf-linux-amd64 && ./shellf-linux-amd64 version
shellf v0.13.0

grep -nE 'v[0-9]+\.[0-9]+\.[0-9]+' README.md returns only the sentence naming v0.10.0 as the
first release carrying SHA256SUMS — a fact about history, which does not drift.

It also downloads one architecture instead of both, which the tagged form did not.

Closes #647