Update installation guide for 8.11

doc/install/installation.md

@@ -269,9 +269,9 @@ sudo usermod -aG redis git
 ### Clone the Source
 
     # Clone GitLab repository
-    sudo -u git -H git clone https://gitlab.com/gitlab-org/gitlab-ce.git -b 8-10-stable gitlab
+    sudo -u git -H git clone https://gitlab.com/gitlab-org/gitlab-ce.git -b 8-11-stable gitlab
 
-**Note:** You can change `8-10-stable` to `master` if you want the *bleeding edge* version, but never install master on a production server!
+**Note:** You can change `8-11-stable` to `master` if you want the *bleeding edge* version, but never install master on a production server!
 
 ### Configure It