ogma-cli: Replace ROS2 with ROS 2 in the README. Refs #83.

The Open Source Robotics Foundation, Inc. (OSRF) has published specific
guidance on how the ROS 2 should be referred to, and they have indicated that
they would want it to be called ROS 2 instead of ROS2.

This commit replaces ROS2 with ROS 2 in Ogma's README.

ogma-cli/README.md

@@ -117,7 +117,7 @@ Available commands:
                            Specification
   fret-reqs-db             Generate a Copilot file from a FRET Requirements
                            Database
-  ros                      Generate a ROS2 monitoring package
+  ros                      Generate a ROS 2 monitoring package
 ```
 
 ## Language transformations: FRET