docs: update undefined var check reference #5216

Job	Run time
build (windows/amd64)	1m 41s
build (freebsd/amd64)	1m 37s
test-freebsd-amd64	3m 7s
test-windows-amd64 (containerd, ./client)	3m 47s
test-windows-amd64 (containerd, ./cmd/buildctl)	3m 9s
test-windows-amd64 (containerd, ./worker/containerd)	2m 25s
test-windows-amd64 (containerd, ./solver)	1m 37s
test-windows-amd64 (containerd, ./frontend)	2m 11s
test-windows-amd64 (containerd, ./frontend/dockerfile)	29m 19s
test-windows-amd64 (./..., 1)	12m 15s
	1h 1m 8s

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