Update README.md

[doowt]

Pull Request

Checklist

• Have you read Digital Catapult's Code of Conduct?
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.

PR Type

Please delete options that are irrelevant.

• Bug Fix
• Chore
• Feature
• Documentation Update
• Code style update (formatting, local variables)
• Breaking Change (fix or feature that would cause existing functionality to change)

Linked tickets

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High level description

This PR corrects the setup instructions for dealing with node-gyp errors.

xcode cannot be installed via brew as was described and xcode-select does not include the required utilities.

Detailed description

N/a

Describe alternatives you've considered

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Operational impact

N/a

Additional context

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