docs: rpm install page - update to centos 7

docs/sources/installation/rpm.md

@@ -52,7 +52,7 @@ Add the following to a new file at `/etc/yum.repos.d/grafana.repo`
 ```bash
 [grafana]
 name=grafana
-baseurl=https://packagecloud.io/grafana/stable/el/6/$basearch
+baseurl=https://packagecloud.io/grafana/stable/el/7/$basearch
 repo_gpgcheck=1
 enabled=1
 gpgcheck=1
@@ -64,7 +64,7 @@ sslcacert=/etc/pki/tls/certs/ca-bundle.crt
 There is also a testing repository if you want beta or release candidates.
 
 ```bash
-baseurl=https://packagecloud.io/grafana/testing/el/6/$basearch
+baseurl=https://packagecloud.io/grafana/testing/el/7/$basearch
 ```
 
 Then install Grafana via the `yum` command.