fix: replace slack.edgexfoundry.org with edgexfoundry.slack.com

[jpwhitemn]

fixes issue 719 in edgex-docs in this repo

Signed-off-by: jpwhitemn [email protected]

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PR Checklist

Please check if your PR fulfills the following requirements:

• I am not introducing a breaking change (if you are, flag in conventional commit message with BREAKING CHANGE: describing the break)
• I am not introducing a new dependency (add notes below if you are)
• I have added unit tests for the new feature or bug fix (if not, why?) n/a
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• I have opened a PR for the related docs change (if not, why?) n/a

Testing Instructions

New Dependency Instructions (If applicable)

[siggiskulason]

LGTM