chore: Remove deprecated description from error_handling/errorsn.rs

[bertonha]

When I was working on error_handling/errorsn.rs I notice that warning.

warning: use of deprecated item 'std::error::Error::description': use the Display impl or to_string()
   --> exercises/error_handling/errorsn.rs:103:49
    |
103 |         f.write_str((self as &dyn error::Error).description())
    |                                                 ^^^^^^^^^^^
    |
    = note: `#[warn(deprecated)]` on by default

[shadows-withal]

👍

[shadows-withal]

@all-contributors add @bertonha for content

[allcontributors]

@fmoko

I've put up a pull request to add @bertonha! 🎉