Update container-registry-helm-repos.md

Using the value for $USER_NAME suggested in the example will lead to this error message:
ERROR: Parameter 'token_name' must conform to the following pattern: '^[a-zA-Z0-9-]*$'.

Updated with a value conforming to the pattern

Author: dips-ogl

articles/container-registry/container-registry-helm-repos.md

@@ -153,7 +153,7 @@ Run  `helm registry login` to authenticate with the registry. You may pass [regi
   ```
 - Authenticate with a [repository scoped token](container-registry-repository-scoped-permissions.md) (Preview).
   ```azurecli
-  USER_NAME="helm-token"
+  USER_NAME="helmtoken"
   PASSWORD=$(az acr token create -n $USER_NAME \
                     -r $ACR_NAME \
                     --scope-map _repositories_admin \