Update slack links

[froggleston]

The Carpentries recently updated their Slack domain URL and invite app URL. This PR updates the links to match the new domains.

[github-actions]

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Rendered Changes

🔍 Inspect the changes: https://github.com/LibraryCarpentry/lc-git/compare/md-outputs..md-outputs-PR-150

The following changes were observed in the rendered markdown documents:

 discuss.md | 2 +-
 md5sum.txt | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

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⏱️ Updated at 2023-08-31 14:33:14 +0000

[emcaulay]

Thank you so much for making these edits! I apologize for not merging it in sooner.