docs: hermetic build README syntax fix #9137

Job	Run time
build (11)	13m 1s
build (17)	13m 2s
build(21) except self-service clients	11m 38s
build(8) except for gapic-generator-java	4m 24s
build(8) for gapic-generator-java	9m 43s
compatibility	1m 30s
lint	28s
dependency_analyzer_unit_test	15s
gapic-generator-java-bom	1m 46s
showcase (11)	4m 25s
showcase (17)	4m 23s
showcase (21)	4m 34s
showcase-clirr	3m 2s
showcase-native	9m 16s
unmanaged_dependency_check	4m 34s
	1h 26m 1s

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