IO::Path method 'extension', argument '$subst' needs an example

[tbrowder]

The docs do show the syntax using method 'extension' with argument '$subst', but they do not show a real use case. I suggest a good example might be:

my $txt-file = "archives.txt".IO;
say $txt-file.basename; # OUTPUT: "archives.txt"

my $pdf-file = $txt-file.extension: "pdf";
say $pdf-file.basename; # OUTPUT: "archives.pdf"