Fix doc for vkuFormatIsXChromaSubsampled #316

Job	Run time
generate_source	18s
chromium	2m 40s
android	35s
build_and_test (Debug, ubuntu-20.04)	1m 43s
reuse	11s
build_and_test (Debug, ubuntu-22.04)	1m 14s
build_and_test (Debug, windows-latest)	2m 32s
build_and_test (Debug, macos-latest)	1m 27s
build_and_test (Release, ubuntu-20.04)	1m 14s
build_and_test (Release, ubuntu-22.04)	1m 20s
build_and_test (Release, windows-latest)	3m 31s
build_and_test (Release, macos-latest)	2m 9s
	18m 54s

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