Switch to use rc.0 in the cronjob tutorial

Should probably use the latest CR and CT in the tutorial.

Author: DirectXMan12

docs/book/src/cronjob-tutorial/testdata/project/Makefile

@@ -57,7 +57,7 @@ docker-push:
 # download controller-gen if necessary
 controller-gen:
 ifeq (, $(shell which controller-gen))
-	go get sigs.k8s.io/controller-tools/cmd/[email protected]beta.3
+	go get sigs.k8s.io/controller-tools/cmd/[email protected]rc.0
 CONTROLLER_GEN=$(shell go env GOPATH)/bin/controller-gen
 else
 CONTROLLER_GEN=$(shell which controller-gen)