Update cache-control.md

[EddiG]

Summary

It seems there is a mistype as the must-revalidate directive affects both the proxy and browser while the proxy-revalidate is only the proxy.

Screenshots (optional)

Documentation checklist

• [ ] The documentation style guide has been adhered to.
• [ ] If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• [ ] Files which have changed name or location have been allocated redirects.