chore: update dev guide to get version of gapic-showcase to install. #5151

Job	Run time
build (11)	11m 57s
build(8) for gapic-generator-java	9m 58s
build (17)	11m 55s
build(8) except for gapic-generator-java	3m 49s
build(21) except self-service clients	11m 31s
compatibility	1m 40s
gapic-generator-java-bom	1m 48s
lint	24s
showcase (11)	4m 56s
showcase-clirr	3m 1s
showcase-native	9m 41s
showcase (17)	4m 58s
unmanaged_dependency_check	4m 9s
	1h 19m 47s

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