Fix mentioned distributions within the setup tutorial
Currently, the PPA from project atomic only works on Ubuntu. We should
only mention that distribution there otherwise it won't work.
@@ -39,7 +39,8 @@ yum install -y \
runc
```
-Debian, Ubuntu, and related distributions:
+On Ubuntu distributions, there is a dedicated PPA provided by
+[Project Atomic](https://www.projectatomic.io/):
```bash
# Add containers-common and cri-o-runc
