Document that when updating using a Helm chart 'helm repo update' needs to be...

Document that when updating using a Helm chart 'helm repo update' needs to be run before the 'helm upgrade ...' command.  

doc/install/kubernetes/gitlab_chart.md

@@ -133,6 +133,7 @@ Once your GitLab Chart is installed, configuration changes and chart updates
 should be done using `helm upgrade`:
 
 ```sh
+helm repo update
 helm upgrade --reuse-values gitlab gitlab/gitlab
 ```