correct signing key information for maven central

[brainbaking]

Hi,

The documentation is wrong with regards to signing information to maven central:

1. You need the shorthand keyid
2. You need the ascii pass in a squashed inline form, devoid of header/footer and newlines

This had me running around in circles for a good few hours. Hopefully someone will not do that after this PR!

Thanks,
Wouter

[asarkar]

I don't think the changes made in this PR are correct. I'm not using the key id, as it's optional, but I'm using the multiline value with the headers as a GitHub Secret, and it works fine in a GitHub action.

[Brutus5000]

Failing with similar problems I had to remove the keyid as well to make it work (now stuck on other problems). so there is something wrong here with the key id at least.

[grimsi]

The key ID has to be exactly the last 8 characters of the full key ID.
I only found this out in the Kotlin documentation which uses this plugin to publish to Maven Central: https://www.jetbrains.com/help/kotlin-multiplatform-dev/multiplatform-publish-libraries.html#add-secrets-to-github

The official docs for this plugin only have the placeholder "12345678" which isn't really obvious IMO.