docs: update beta install command

[ankurk91]

Description

This will help people who are migrating to vue 3

Checklist

• [ ] Tests
• [x] Documentation
• [ ] Used commitizen and followed Conventional Commits
• [ ] Commit footer references issue num. If applicable.

[github-actions[bot]]

:tada: This PR is included in version 3.0.0-beta.2 :tada:

The release is available on:

• npm package (@beta dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: