Add namespace override example

Adding namespace override to the examples when
there is dev in the stack.yml and during deployment
you override the namespace with the cli

Signed-off-by: Martin Dekov <mvdekov@gmail.com>

Author: martindekov

docs/reference/namespaces.md

@@ -102,3 +102,11 @@ faas-cli deploy
 
 head -c 16 /dev/urandom | faas-cli invoke --namespace dev stronghash
 ```
+
+Override the namespace configured in the stack.yml when deploying the function and test:
+
+```sh
+faas-cli deploy --namespace staging-fn
+
+head -c 16 /dev/urandom | faas-cli invoke --namespace staging-fn stronghash
+```