Make default namespace in kustomize match the docs

Signed-off-by: Adam Reese <adam@reese.io>

config/default/kustomization.yaml

@@ -1,5 +1,5 @@
 # Adds namespace to all resources.
-namespace: spin-operator-system
+namespace: spin-operator
 
 # Value of this field is prepended to the
 # names of all resources, e.g. a deployment named