fix: genericize port number for proxy

[mjang]

Proposed changes

As web proxy servers do not have a "standard" port, it's helpful to make the number more generic. The current doc specifies 3128, which implies a specific proxy server.

This PR replaces that number with <port number>

Checklist

Before sharing this pull request, I completed the following checklist:

• I read the Contributing guidelines
• My branch adheres to the Git conventions
• My content changes adhere to the F5 NGINX Documentation style guide
• If my changes involve potentially sensitive information¹, I have assessed the possible impact
• I have waited to ensure my changes pass tests, and addressed any discovered issues

Footnotes

1. Potentially sensitive information includes personally identify information (PII), authentication credentials, and live URLs. Refer to the style guide for guidance about placeholder content. ↩

[github-actions]

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