Update README.md link for slack-go library

[adlai]

From the old nlopes/slack README file:

You're encouraged to update your import statements to use the official slack-go/slack version above.

It looks like the code was already updated and only this link was overlooked.

[qlty-cloud-legacy]

Code Climate has analyzed commit 84f1d7e and detected 0 issues on this pull request.

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[adlai]

Are the golangci-lint and test-build-upload really critical for this PR? It only modifies one link, in the human-targeted documentation. Ideally those actions would not be required for such patches, or at least would detect that the patch doesn't affect source code.