give --version the success exit code

[QJKX]

Change exit code of jamulus --version from 1 to 0.

Printing version information as requested is not a failure, so by convention the program should exit with an exit code of 0.
A non-zero exit code would interrupt a shell script following the convention.

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I've filled all the content above