Change WSGI_PROXIES to PROXYFIX_CONFIG in documentation

[tmorrell]

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Description

We've already noted that WSGI_PROXIES is going away 2edae42, so this is just updating another mention.

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• I'm aware of the code of conduct.
• I've created logical separate commits and followed the commit message format.
• I've targeted the master branch.
• If this documentation change impacts the current release of InvenioRDM, I will backport it to the production branch following approval or indicate to a maintainer that it should be backported.

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