Corrected namespaceSelector for network policy

Corrected namespaceSelector for intra-namespace network policy
example. The label "name" does not exist on the namespace resource
kube-system, but the label "kubernetes.io/metadata.name" does.

Signed-off-by: Lars Wefald <larwef@gmail.com>

docs/security/hardening-guide.md

@@ -380,7 +380,7 @@ spec:
     - from:
       - namespaceSelector:
           matchLabels:
-            name: kube-system
+            kubernetes.io/metadata.name: kube-system
 ```
 
 With the applied restrictions, DNS will be blocked unless purposely allowed. Below is a network policy that will allow for traffic to exist for DNS.