Fixed links in `x/checkpointing/README.md` (#731) #40

Job	Run time
docker_pipeline / docker_build	2m 51s
lint_test / build	37s
lint_test / integration-tests	18m 14s
lint_test / lint	3m 2s
lint_test / check-mock-gen	0s
lint_test / unit-tests	5m 11s
docker_pipeline / dockerhub_publish	0s
docker_pipeline / ecr_publish	0s
	29m 55s

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