docs: update undefined var check reference #4662

Job	Run time
prepare	1s
test / prepare	1m 52s
test / run (containerd, ./frontend/dockerfile, integration)	4m 52s
test / run (containerd, ./frontend/dockerfile, dockerfile)	7m 13s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	4m 44s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	6m 23s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	4m 51s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	7m 15s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 2s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	7m 8s
test / run (oci, ./frontend/dockerfile, integration)	4m 51s
test / run (oci, ./frontend/dockerfile, dockerfile)	5m 33s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 51s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	6m 6s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	4m 34s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	6m 11s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 17s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	6m 16s
image (mainline)	5m 12s
image (labs)	4m 58s
scout	0s
release	0s
	1h 42m 10s

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