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.

[clalancette]

Thanks for the contribution.

Can you please keep the indentation the same for the parts of the code that don't change? That will make this easier to review.

Also, CI is failing because we don't allow trailing whitespace. If you fix that, we can review this. Thanks.