Update Custom-ROS2-Interfaces.rst

[followthwhiterabbit]

I have changed the executable file's name inside add_executable in CmakeLists.txt file. Because the files were from the previous sum for 2 arguments from the console but the files we updated were designed for three items.

[followthwhiterabbit]

It may not be worth it, yes, you're right, I am following the tutorials and had a build error problem, most of the people who follow the documentation should have found the solution already, I am just being meticulous

[clalancette]

I'm going to close this one out for now, since I don't think we are going to merge this as-is. Thanks for the contribution, though.