Fix docsrs linkification for kebab case package names #147

When generating the docsrs link for a package, miette currently generates a URL like this:

• https://docs.rs/nu-protocol/0.60.0/nu-protocol/enum.ShellError.html#variant.DivisionByZero

But the correct URL is to convert the second name to that of its rust module name, which converts - to _:

• https://docs.rs/nu-protocol/0.60.0/nu_protocol/enum.ShellError.html#variant.DivisionByZero

This is making it so navigating to docsrs links instead redirects to, for example:

• https://docs.rs/nu-protocol/0.60.0/nu_protocol/?search=enum.ShellError.html#variant.DivisionByZero

which is broken!