Use gitlab-workhorse 0.6.3 in installation guide

doc/install/installation.md

@@ -355,7 +355,7 @@ GitLab Shell is an SSH access and repository management software developed speci
     cd /home/git
     sudo -u git -H git clone https://gitlab.com/gitlab-org/gitlab-workhorse.git
     cd gitlab-workhorse
-    sudo -u git -H git checkout 0.6.2
+    sudo -u git -H git checkout 0.6.3
     sudo -u git -H make
 ### Initialize Database and Activate Advanced Features