Misc typos on the Installing Percona Server for MySQL on Red Hat Enterprise Linux and CentOS page

Found few typos and incorrect formatting issues on the CentOS/RHEL installation doc page:

• . _yum_repo: at the top of the page should probably be .. _yum_repo:
• Repo installation starts with a lowercase and probably is missing P.
• Note about the RHEL 8 package installation is broken and doesn't contain any information about the command you should run to disable the module