Update the note about custom SSL certificates in README.md

[SaswatPadhi]

I get the following error when trying to use my custom privkey.pem and fullchain.pem:

unifi-1  | [2024-11-29 00:32:16,446] <docker-entrypoint> WARN - Custom SSL: missing '/usr/lib/unifi/cert/cert.pem'
unifi-1  | [2024-11-29 00:32:16,458] <docker-entrypoint> WARN - UniFi v7.3+ Custom SSL: *********** '/usr/lib/unifi/cert/fullchain.pem' IS DEPRECATED ***********
unifi-1  | [2024-11-29 00:32:16,470] <docker-entrypoint> WARN - UniFi v7.3+ Custom SSL: Use a single / non-chain cert file named '/usr/lib/unifi/cert/cert.pem' moving forward
unifi-1  | [2024-11-29 00:32:16,483] <docker-entrypoint> WARN - UniFi v7.3+ Custom SSL: Failure to update current setup may result in constant <ble-load-keystore> errors
unifi-1  | [2024-11-29 00:32:16,497] <docker-entrypoint> WARN - Custom SSL: certificate import was NOT performed

Apparently, Unifi app versions 7.3+ expect a different filename for the custom certificate.

@goofball222

[goofball222]

Thanks! I missed updating the documentation when that changed.