docs: update packaging notation for local development requirements

[zimeg]

Summary

This PR updates the maintainers_guide.md packaging step for local build developments to match a recent change to the slack_sdk notes 📚

Testing

An expanded version of these line can be found with pip freeze!

Category

• Document pages under /docs
• maintainers_guide.md
• Others

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run ./scripts/install_all_and_run_tests.sh after making the changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.96%. Comparing base (9c26945) to head (2cc020e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #1271 +/- ##
=======================================
Coverage 90.96% 90.96% =======================================
Files 222 222 Lines 7501 7501 =======================================
Hits 6823 6823 Misses 678 678 

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[WilliamBergamin]

Nice 💯 thanks for standardizing this 🥇