benchmarks: update readme.txt
benchmarks: update readme.txt

Remove references to google_speed.proto which was removed in
commit 2e83110230b7.

Also fix-up commands to build and run.

google/protobuf,Livefyre/protobuf,google/protobuf,google/protobuf,Livefyre/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,Livefyre/protobuf,google/protobuf,Livefyre/protobuf,google/protobuf,Livefyre/protobuf