Fixing link to CODE_OF_CONDUCT.md within README.md

[codylewis]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?

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On https://github.com/Homebrew/brew/blob/master/README.md the link to the Code of Conduct page leads to a 404 page.

Updating the link allows a user to navigate to the correct page to view the resource.

Same issue as #3266

[MikeMcQuaid]

Thanks for your first contribution to Homebrew! Without people like you submitting PRs we couldn't run this project. You rock!