Fix doc for vkuFormatIsXChromaSubsampled #313

Job	Run time
chromium	2m 9s
android	29s
generate_source	18s
build_and_test (Debug, ubuntu-20.04)	1m 30s
build_and_test (Debug, ubuntu-22.04)	1m 2s
reuse	13s
build_and_test (Debug, windows-latest)	2m 41s
build_and_test (Debug, macos-latest)	1m 34s
build_and_test (Release, ubuntu-20.04)	1m 14s
build_and_test (Release, ubuntu-22.04)	1m 23s
build_and_test (Release, windows-latest)	2m 46s
build_and_test (Release, macos-latest)	1m 47s
	17m 6s

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