Update the `join` command format in the k8s Running Consul section

Based on info from consul-helm issue 16, the formatting of the helm
chart value for joining an external cluster needs to be specified
as a yaml array. This updates the documentation to reflect this.

website/source/docs/platform/k8s/run.html.md

@@ -94,7 +94,9 @@ global:
 client:
   enabled: true
   join:
-    - "provider=my-cloud config=val ..."
+    - "provider=my-cloud"
+    - "config=val"
+    - "..."
 ```
 
 The `values.yaml` file to configure the Helm chart sets the proper