chore: Update README.md to fix installation instructions

[julieqiu]

Fix this error in the installation instructions:

go install github.com/google/gnostic/cmd/protoc-gen-openapi
go: 'go install' requires a version when current directory is not in a module
	Try 'go install github.com/google/gnostic/cmd/protoc-gen-openapi@latest' to install the latest version