docs: Update gRPC spelling in READMEs #2966

Job	Run time
filter	1m 31s
sdk-platform-java units (8)	3m 50s
sdk-platform-java integration (25)	10m 21s
dependency_analyzer_unit_test	1m 25s
sdk-platform-java self-service clients (17)	9m 35s
dependencies	3m 37s
sdk-platform-java integration (11)	10m 20s
sdk-platform-java integration (21)	10m 19s
sdk-platform-java units (17)	4m 16s
sdk-platform-java self-service clients (11)	9m 38s
java8-compatibility	6m 7s
clirr	3m 50s
sdk-platform-java integration (17)	10m 27s
sdk-platform-java units (25)	4m 14s
unmanaged_dependency_check	11m 11s
sdk-platform-java units (11)	4m 41s
gapic-generator-java (8)	11m 32s
gapic-generator-java-bom	3m 19s
sdk-platform-java units (21)	4m 14s
	2h 4m 27s

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