Update Deployment.md

Used non existing variable in setContainers. Possibly copied from the test.

Author: svenvanhees

docs/kinds/Deployment.md

@@ -18,7 +18,7 @@ $container = K8s::container()
 $pod = K8s::pod()
     ->setName('mysql')
     ->setLabels(['tier' => 'backend']) // needs deployment-name: mysql so that ->getPods() can work
-    ->setContainers([$mysql]);
+    ->setContainers([$container]);
 
 $dep = $cluster->deployment()
     ->setName('mysql')