Enable gpg checks in installation examples

[ac000]

Hi,

I was just wondering if there was a specific reason that the install documentation has the GPG check disabled for the RHEL repo config?

I've added the below to my unit.repo and it allowed me to install without issue and seems a pretty reasonable thing to have enabled? :)

gpgkey=https://unit.nginx.org/keys/nginx-keyring.gpg
gpgcheck=1

Originally posted by @RielDowner in nginx/unit#1389