Fixes rustdoc::bare_urls lint

[GuillaumeGomez]

It fixes this:

warning: this URL is not a hyperlink
 --> src/utils.rs:3:5
  |
3 | //! https://github.com/BenjaminRi/Redwood-Wiki/blob/master/src/markdown_utils.rs.
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use an automatic link instead: `<https://github.com/BenjaminRi/Redwood-Wiki/blob/master/src/markdown_utils.rs.>`
  |
  = note: bare URLs are not automatically turned into clickable links
  = note: `#[warn(rustdoc::bare_urls)]` on by default

warning: this URL is not a hyperlink
 --> src/utils.rs:5:5
  |
5 | //! https://github.com/raphlinus/pulldown-cmark/issues/507.
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use an automatic link instead: `<https://github.com/raphlinus/pulldown-cmark/issues/507.>`
  |
  = note: bare URLs are not automatically turned into clickable links

warning: `pulldown-cmark` (lib doc) generated 2 warnings (run `cargo fix --lib -p pulldown-cmark` to apply 2 suggestions)

[Martin1887]

Good catch, thanks!