Fix link in design doc readme (#433)

Author: Skarlso

docs/design/README.md

@@ -3,7 +3,7 @@
 - [Constraints](./constraints.md)
 - [Health](./health.md)
 - [Metrics](./metrics.md)
-- [Kubernetes Pod name versus cluster ID](./pod_names_versus_cluster_id.md)
+- [Kubernetes Pod name versus cluster ID](./pod_name_versus_cluster_id.md)
 - [Resource & labels](./resource_and_labels.md)
 - [Scaling](./scaling.md)
 - [Status](./status.md)