docs/installing-vernemq/centos_and_redhat macro issues

[bby-bishopclark]

On https://docs.vernemq.com/installing-vernemq/centos_and_redhat :

• vernemq-<%= latest_version() %>-1.el7.centos.x86_64.rpm looks weird

It looks like a macro blew up.

We don't need to specify versions with yum anyway - when the version we want is the latest - as it will just go get the latest from the yum repo. So, we could just say sudo yum install vernemq and let it just work.

• the yum repo URL is missing

It's customary to have a yum repo while installing via yum; otherwise it'd just be an rpm invocation. Either way, we need a URL (or instructions how to find the URL!).

(And I wouldn't recommend just pointing to bare RPMs, as that's a very old way of doing that; since yum automatically checks for updates, it's a much better way of installing RPMs and keeping them updated. If we see how many 'supply chain' security issues we have [oh, hi npm] these days, then we realize we want yum instead of just rpm)