kubernetes: Fix Federation example in README (#1515)

Remove `fallthrough` from the federation example in the README.  `fallthrough` isn't needed here and has no effect (federation has an implicit fallthrough for non-federated queries).  
It's inclusion is confusing.

plugin/kubernetes/README.md

@@ -167,10 +167,8 @@ feature enables serving federated domains from the kubernetes clusters.
 
     cluster.local {
         federation {
-            fallthrough
             prod prod.example.org
             staging staging.example.org
-
         }
         kubernetes
     }