etcd: world: Add missing defer

Not sure how this snuck in!
I've now ran a quick grep across the code base, and I can't find any
similar mistakes.

ack '.Done()' | grep -v defer | grep -iv ctx # then check these

etcd/world.go

@@ -76,7 +76,7 @@ func (obj *World) IdealClusterSizeWatch(ctx context.Context) (chan error, error)
 	}
 	util.WgFromCtx(ctx).Add(1)
 	go func() {
-		util.WgFromCtx(ctx).Done()
+		defer util.WgFromCtx(ctx).Done()
 		// This must get closed *after* because it will not finish until
 		// the Watcher returns, because it contains a wg.Wait() in it...
 		defer c.Close() // ignore error