[Issue] Update CONTRIBUTING.md

[m2-assistant]

This issue is automatically created based on existing pull request: #33163: Update CONTRIBUTING.md

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Adding correct path for Forks And Pull Requests Instructions. Added updated link for beginners Slack chat, Which will redirect the user to the Slack Application instead of opening the Slack Channel in the browser. If the user is not having a Slack Application, then beginners Slack chat will open in the browser.

Description (*)

Related Pull Requests

Fixed Issues (if relevant)

1. Fixes magento/magento2#<issue_number>

Manual testing scenarios (*)

1. ...
2. ...

Questions or comments

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• README.md files for modified modules are updated and included in the pull request if any README.md predefined sections require an update
• All automated tests passed successfully (all builds are green)