Fix typo in GatewayClassConfig docs #11129

Job	Run time
Get files changed and conditionally skip CI	38s
noop	1s
Setup	0s
check-go-mod	0s
check-codegen	0s
check-generated-protobuf	0s
lint-enums	0s
lint	0s
lint-consul-retry	0s
lint-container-test-deps	0s
dev-build	0s
lint-32bit	0s
go-test-ce	0s
go-test-enterprise	0s
go-test-race	0s
go-test-32bit	0s
go-test-envoyextensions	0s
go-test-api-1-19	0s
go-test-api-1-20	0s
go-test-troubleshoot	0s
go-test-sdk-1-19	0s
go-test-sdk-1-20	0s
go-tests-success	0s
	39s

Provide feedback