Add libxml2 install command for macOS in contributor quick start doc

[yysd5]

The both documents below states that libxml2 should be installed as a required package for the pre-commit hook.
While the first document includes installation commands for both Linux and Mac, the next document lacks instructions for Mac. So, I have added the Mac installation commands here.

https://github.com/apache/airflow/blob/main/STATIC_CODE_CHECKS.rst#pre-commit-hooks
https://github.com/apache/airflow/blob/main/CONTRIBUTORS_QUICK_START.rst#configuring-pre-commit

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^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in newsfragments.

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/main/CONTRIBUTING.rst)
Here are some useful points:

• Pay attention to the quality of your code (ruff, mypy and type annotations). Our pre-commits will help you with that.
• In case of a new feature add useful documentation (in docstrings or in docs/ directory). Adding a new operator? Check this short guide Consider adding an example DAG that shows how users should use it.
• Consider using Breeze environment for testing locally, it's a heavy docker but it ships with a working Airflow and a lot of integrations.
• Be patient and persistent. It might take some time to get a review or get the final approval from Committers.
• Please follow ASF Code of Conduct for all communication including (but not limited to) comments on Pull Requests, Mailing list and Slack.
• Be sure to read the Airflow Coding style.
• Always keep your Pull Requests rebased, otherwise your build might fail due to changes not related to your commits.
  Apache Airflow is a community-driven project and together we are making it better 🚀.
  In case of doubts contact the developers at:
  Mailing List: dev@airflow.apache.org
  Slack: https://s.apache.org/airflow-slack

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.

[hussein-awala]

Congrats on your first commit 🎉

[yysd5]

Congrats on your first commit 🎉

@hussein-awala
Thank you!!
Even though it was just a small fix to the doc, I'm happy to have made my first contribution to Airflow, which I always use🙏
If there are other issues that seem within my capacity, I'll try to tackle them.