Update gitlab-shell version to 3.2.1 in the 8.9->8.10 update guide

This was missing from gitlab-org/gitlab-ce!5509.
Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/update/8.9-to-8.10.md

@@ -46,7 +46,7 @@ sudo -u git -H git checkout 8-10-stable-ee
 ```bash
 cd /home/git/gitlab-shell
 sudo -u git -H git fetch --all --tags
-sudo -u git -H git checkout v3.2.0
+sudo -u git -H git checkout v3.2.1
 ```
 
 ### 5. Update gitlab-workhorse