Reference slack, not mattermost in README

[yuvipanda]

What does this PR do?

The invite is being sent to a slack channel, not a Mattermost channel

Screenshot/screencast of this PR

What issues does this PR fix or reference?

How to test this PR?

• Click the link
• Notice it's sending a slack invite

PR Checklist

As the author of this Pull Request I made sure that:

• [x] The Eclipse Contributor Agreement is valid
• [x] Code produced is complete
• [x] Code builds without errors
• [x] Tests are covering the bugfix or new feature
• [x] The repository devfile is up to date and works
• [x] Sections What issues does this PR fix or reference and How to test this PR completed
• [x] Relevant user documentation updated
• [x] Relevant contributing documentation updated
• [x] CI/CD changes implemented, documented and communicated

Reviewers

Reviewers, please comment how you tested the PR when approving it.

[yuvipanda]

I signed the agreement!