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]

Thank you!

Thank you for your pull request 😃

🤖 This automated message can help you check the rendered files in your submission for clarity. If you have any questions, please feel free to open an issue in {sandpaper}.

If you have files that automatically render output (e.g. R Markdown), then you should check for the following:

• 🎯 correct output
• 🖼️ correct figures
• ❓ new warnings
• ‼️ new errors

Rendered Changes

🔍 Inspect the changes: https://github.com/LibraryCarpentry/lc-spreadsheets/compare/md-outputs..md-outputs-PR-146

The following changes were observed in the rendered markdown documents:

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

What does this mean?

If you have source files that require output and figures to be generated (e.g. R Markdown), then it is important to make sure the generated figures and output are reproducible.

This output provides a way for you to inspect the output in a diff-friendly manner so that it's easy to see the changes that occur due to new software versions or randomisation.

⏱️ Updated at 2023-08-31 14:35:49 +0000

[morskyjezek]

Thank you for the change!