Fix: clarify it's needed only for Centos 6 and 7

Author: alanfranz

centos/content.md

@@ -22,7 +22,7 @@ For example: `docker pull centos:5.11` or `docker pull centos:6.6`
 
 ## Overlayfs and yum
 
-Recent Docker versions support the [overlayfs](https://docs.docker.com/engine/userguide/storagedriver/overlayfs-driver/) backend, which is enabled by default on most distros supporting it from Docker 1.13 onwards. **This backend requires yum-plugin-ovl to be installed and enabled**; while it is installed by default in recent centos images, make it sure you retain the ```plugins=1``` option in ```/etc/yum.conf``` if you update that file; otherwise, you may encounter errors related to rpmdb checksum failure - see [docker ticket 10180](https://github.com/docker/docker/issues/10180) for more details.
+Recent Docker versions support the [overlayfs](https://docs.docker.com/engine/userguide/storagedriver/overlayfs-driver/) backend, which is enabled by default on most distros supporting it from Docker 1.13 onwards. On Centos 6 and 7, **that backend requires yum-plugin-ovl to be installed and enabled**; while it is installed by default in recent centos images, make it sure you retain the ```plugins=1``` option in ```/etc/yum.conf``` if you update that file; otherwise, you may encounter errors related to rpmdb checksum failure - see [docker ticket 10180](https://github.com/docker/docker/issues/10180) for more details.
 
 # Package documentation