Update README.md to point to #fenix rather than #focus

[KWierso]

I feel like this was just mistyped (or a carryover from when Focus was the only geckoview browser, maybe?), because the link to view chat logs was already pointing to #fenix

Pull Request checklist

• [This only changes a single word in the readme file. If this broke linters, I'd be worried.] Quality: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
• [Only changed a single word in the readme] Tests: This PR includes thorough tests or an explanation of why it does not
• [Don't think this warrants a changelog entry.] Changelog: This PR includes a changelog entry or does not need one
• [Changing a single word in the readme file shouldn't make this any less accessible.] Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features

[KWierso]

because the link to view chat logs was already pointing to #fenix

Excitingly, #fenix is not currently logged by logbot, so the link to view logs doesn't work!