Update README.md

Author: Ravi Prakash Joshi

lib/README.md

@@ -1,5 +1,5 @@
 # Note
-Older version of `protobuf` might be installed already. Hence, in order to ignore the conflict, we are using locally install `protobuf`
+Older versions of `protobuf` might be installed already. Hence, in order to ignore the conflict, we are using locally installed `protobuf`
 
 # Steps
 1. Download the `protobuf-3.5.0.tar.gz` from release page. The URL of the file is [here](https://github.com/ravijo/ros_protocol_buffer_tutorial/releases/download/1/protobuf-3.5.0.tar.gz)