Update Custom-ROS2-Interfaces.rst

[EliottFerry]

Changing python codeblock to match previous AddThreeInts implementation (C++ might need a check also, i followed only the Python tutorial)

[kscottz]

@EliottFerry can you clarify what you mean by, "match previous AddThreeInts implementation."

[EliottFerry]

@kscottz Of course, on this step, you have the implementation of MinimalClientAsync along with a main function. This main implementation does not match with this implementation, where some error handling is being implemented, without the #CHANGE tags showing the line that as been changed.
I got "confused" reaching this step, since I did not understood why the main function had changed that much. Maybe the second implementation should be the one to be kept to give an example of error handling.