update Helm docs with latest behaviour

Update readme to address regression bug that doesn't automatically
trigger HelmRelease deployment if
-> the full Docker Registry URL + image name + image label are not provided
  in a Helm Chart as a single variable (not splitted)
-> which in turn is read by the HelmRelease

Resolves:
Related: fluxcd#2067
Signed-off-by: Daniel Andrei Minca <mandrei17@gmail.com>

site/helm-get-started.md

@@ -160,9 +160,7 @@ change the `tag:` line to the following:
 
 ```yaml
   values:
-    image:
-      repository: bitnami/mongodb
-      tag: 4.0.6
+    image: bitnami/mongodb:4.0.6
 ```
 
 Commit the change to your `master` branch. It will now get