Fix some formatting in protobuf spec.

Change-Id: I8aba128dad1109d6740710498e69b732929164d1
Reviewed-on: http://review.couchbase.org/42848
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>