Update installation.md

fixed branch-name (5-3 -> 5-4)

Author: Lukas Schauer

doc/install/installation.md

@@ -150,7 +150,7 @@ To setup the MySQL/PostgreSQL database and dependencies please see [`doc/install
     cd /home/git/gitlab
 
     # Checkout to stable release
-    sudo -u git -H git checkout 5-3-stable
+    sudo -u git -H git checkout 5-4-stable
 
 **Note:**
 You can change `5-3-stable` to `master` if you want the *bleeding edge* version, but do so with caution!