ntfy: Set instance label for Argo CD

Argo CD wants every resource managed by an application to have that
application's name as the value of the `app.kubernetes.io/instance`
label.

ntfy/ntfy.yaml

@@ -2,6 +2,8 @@ apiVersion: v1
 kind: Namespace
 metadata:
   name: ntfy
+  labels:
+    app.kubernetes.io/instance: ntfy
 
 ---
 apiVersion: v1