fix: Remove contributing docs referencing beta branch

[AshCorr]

What does this change?

Removes the docs referencing the beta branch, it looks like this only worked before we updated to changesets as using different release channels is a semantic-release feature

[changeset-bot[bot]]

⚠️ No Changeset found

Latest commit: 42b0b24d6296038582b445f4c8689d2c363c26ed

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[AshCorr]

Thanks! Shame we can't replicate this behaviour with changesets.

With a bit of work we could probably achieve a similar result, just not as straightforward anymore.