[ci-skip] Correct gitlab-shell installation instructions in docs

With the introduction of repository storages validations it becomes
necessary to add the flag `SKIP_STORAGE_VALIDATION` to the gitlab-shell
install command, since that command will create the storage paths

doc/install/installation.md

@@ -381,7 +381,7 @@ sudo usermod -aG redis git
 GitLab Shell is an SSH access and repository management software developed specially for GitLab.
 
     # Run the installation task for gitlab-shell (replace `REDIS_URL` if needed):
-    sudo -u git -H bundle exec rake gitlab:shell:install REDIS_URL=unix:/var/run/redis/redis.sock RAILS_ENV=production
+    sudo -u git -H bundle exec rake gitlab:shell:install REDIS_URL=unix:/var/run/redis/redis.sock RAILS_ENV=production SKIP_STORAGE_VALIDATION=true
 
     # By default, the gitlab-shell config is generated from your main GitLab config.
     # You can review (and modify) the gitlab-shell config as follows: