docs: Fix outdated slack notification configuration readme

[syscl]

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Toolchain Guide
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

[gdsoumya]

You probably need to update the docs in the notifications package https://github.com/argoproj/notifications-engine and then update go mod to include the version of notifications that contains our change and run codegen to update argocd docs.

[pasha-codefresh]

@syscl are you going to work on it ?

[syscl]

Hi @pasha-codefresh thank you for reminding, yes. Any extra steps still needs for the PR?

[pasha-codefresh]

You need to do these changes inside notification-engine and after update notification-engine version inside go.mod