[AIRFLOW-XXXX] Fix email configuration link in CONTRIBUTING.rst

[coopergillan]

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Issue link: Document only change, no JIRA issue

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Update the link to the email configuration in CONTRIBUTING.rst.

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• Description above provides context of the change
• Commit message/PR title starts with [AIRFLOW-NNNN]. AIRFLOW-NNNN = JIRA ID^*
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[mik-laj]

Hi. This change seems valuable to me. Why did you close this PR?