Update mail-proxy.md

[theJorge]

Removed gerund from title according to style guide.

Proposed changes

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[github-actions]

✅ All required contributors have signed the F5 CLA for this PR. Thank you!
_{Posted by the CLA Assistant Lite bot.}

[theJorge]

I have hereby read the F5 CLA and agree to its terms

[mjang]

Thank you for your successful contribution!