Corrects the update document to match the version of gitshell specified in the...

Corrects the update document to match the version of gitshell specified in the install document (1.7.1)

doc/update/6.0-to-6.1.md

@@ -36,7 +36,7 @@ sudo -u git -H git checkout 6-1-stable
 ```bash
 cd /home/git/gitlab-shell
 sudo -u git -H git fetch
-sudo -u git -H git checkout v1.7.0
+sudo -u git -H git checkout v1.7.1
 ```
 ### 4. Install libs, migrations, etc.