doc: Fix stunner namespace in the magic mirror example

Without this, stunner was installed in the default namespace, while the
configuration file "kurento-magic-mirror-stunner.yaml" expects it to be
in the "stunner" namespace.

This caused 'namespace "stunner" not found' errors when following the
demo steps and then configuring stunner by applying
"kurento-magic-mirror-stunner.yaml".

By installing stunner in the "stunner" namespace, the demo installation
works as expected.

docs/examples/kurento-magic-mirror/README.md

@@ -37,7 +37,7 @@ Install the STUNner gateway operator and STUNner ([more info](https://github.com
 helm repo add stunner https://l7mp.io/stunner
 helm repo update
 helm install stunner-gateway-operator stunner/stunner-gateway-operator --create-namespace --namespace=stunner-system
-helm install stunner stunner/stunner
+helm install stunner stunner/stunner --create-namespace --namespace=stunner
 ```
 
 Install the WebRTC application and Kurento media servers, altogether with the corresponding services and Kubernetes objects (see the content of the yaml for details):
@@ -82,4 +82,4 @@ $ kubectl scale deployment kms --replicas=4
 
 # Help
 
-STUNner development is coordinated in Discord, feel free to [join](https://discord.gg/DyPgEsbwzc).
+STUNner development is coordinated in Discord, feel free to [join](https://discord.gg/DyPgEsbwzc).