metropolis: remove stutter in ClusterConfiguration.KubernetesConfig

We already know this is a config (it lives in ClusterConfiguration), no
need to call that a config again.

This doesn't break any compatibility yet as field names are not (yet)
under a stability guarantee.

Change-Id: Ib6492d1c8303cbd0620b979b8047ec9757e301c0
Reviewed-on: https://review.monogon.dev/c/monogon/+/3594
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
Tested-by: Jenkins CI
diff --git a/metropolis/proto/api/management.proto b/metropolis/proto/api/management.proto
index 0302a79..30f41c1 100644
--- a/metropolis/proto/api/management.proto
+++ b/metropolis/proto/api/management.proto
@@ -531,7 +531,7 @@
   // if set) into the new config state.
   //
   // Currently, only the following fields can be mutated:
-  //   1. kubernetes_config.node_labels_to_synchronize
+  //   1. kubernetes.node_labels_to_synchronize
   google.protobuf.FieldMask update_mask = 3;
 }