doc: Replace CentOS with Rocky / Alma

Unfortunately Red Hat's CentOS is not longer an open alternative to
RHEL, this replace it in the setup instructions with Rock / Alma Linux.

Signed-off-by: Nicolai Buchwitz <nb+github@tipi-net.de>

README.md

@@ -17,9 +17,9 @@ This check command depends on **Python 3** and the following modules:
 apt install python3 python3-requests python3-packaging
 ```
 
-**Installation on Redhat 7 / CentOS 7**
+**Installation on Rocky / Alma Linux 9**
 ```
-yum install python36 python36-requests python36-packaging
+yum install python3 python3-requests python3-packaging
 ```
 
 **Installation on FreeBSD**