docs(webpki-roots): simplify usage example

[robjtede]

copy-pasting the main usage example produces a clippy warning suggesting this instead

[cpu]

CI failure is unrelated. Looks like there was an upstream data update between yesterday's cron run and this branch's run. We'll fix that separately.

[cpu]

CI failure is unrelated.

Ah, my mistake. It is related since the example is sourced from the template in the tooling it needs to be updated as well. @robjtede Can you update this place as well: https://github.com/rustls/webpki-roots/blob/7edc9159d7829e72f5409ce13b66881d60b40ffe/webpki-roots/tests/codegen.rs#L142-L145

[ctz]

Thanks!