docs: change invalid RFC3339 link for valid one

[char0n]

https://xml2rfc.ietf.org/public/rfc/html/rfc3339.html now returns 404. This PR replaces the no longer valid link for another equivalent one: https://www.rfc-editor.org/rfc/rfc3339.html#section-5.6

Refs #831

[char0n]

@fmvilas @derberg @dalelane can you please look at this editorial change? This is currently blocking effort of updating the release branch - #831. Thanks!

[derberg]

@char0n I changed title to docs: and fix(spec): would trigger a patch release after merge.

docs: is critical for small editorial changes, and then we push it out also directly to the website -> asyncapi/website#938

[sonarqubecloud]

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[derberg]

/rtm

not waiting for @fmvilas as he is aware of the issue and tried to fix it on his PR -> https://github.com/asyncapi/spec/pull/827/files#r968898025

[asyncapi-bot]

🎉 This PR is included in version 2.5.0-next-major-spec.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[fmvilas]

Forget about the last comment saying it was released in version 2.5.0-next-major-spec.1. I made a mistake and it created this version but it should actually be 3.0.0-next-major-spec.1. There's a notice in the release. Apologies for the noise.

[asyncapi-bot]

🎉 This PR is included in version 2.5.0-next-spec.5 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[derberg]

Recent comments about the release from the bot were added by mistake. More details in #899