Update README.md

[patrick-emmanuel]

Update broken link

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

PR resolves issue with broken link in the docs: #5685

[apollo-cla]

@the-bionic: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[benjamn]

Thanks for catching this! Since Apollo Boost is deprecated (and this file no longer exists) in AC3, I believe we no longer need this change, though please let us know if you find other broken links (Boost-related or otherwise).