docs: update readme yarn version

[mhuynh5757]

Submission Checklist

• [x] The README.md reflects new features
• [x] The CHANGELOG.md reflects any feature, bug fixes, or known issues made in the source code
• [x] New unit tests and integration tests have beed added
• [x] Code coverage is more or equal to the previous coverage
• [x] A visual inspection of the Files changed tab was made prior to submitting the pull request ensuring the style guide was followed
• [x] CI pipeline is green
• [x] Included screenshot if necessary

Description

This PR updates the yarn version referenced in the readme to be the one used when testing locally and used in CI.

Breakdown

• N/A

Validation

• N/A

Additional Notes

N/A

Contributing to Twilio

All third-party contributors acknowledge that any contributions they provide will be made under the same open-source license that the open-source project is provided under.

• [x] I acknowledge that all my contributions will be made under the project's license.