util.prosodyctl.cert: Pass variables via formatting instead of concatenation
Prevents potential weirdness in case there's any %s or such in a host,
file or directory name, since show_warning() is printf().
Thanks for your interest in contributing to the project!
There are many ways to contribute, such as helping improve the
documentation, reporting bugs, spreading the word or testing the latest
development version.
You can find more information on how to contribute at <https://prosody.im/doc/contributing>
See also the HACKERS and README files.