Fix bare url warnings in multipart docs

[nickelc]

The comments are not intended as doc comments.

Warnings

warning: this URL is not a hyperlink
   --> src/async_impl/multipart.rs:411:5
    |
411 | /// https://url.spec.whatwg.org/#fragment-percent-encode-set
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use an automatic link instead: `<https://url.spec.whatwg.org/#fragment-percent-encode-set>`
    |
    = note: `#[warn(rustdoc::bare_urls)]` on by default
    = note: bare URLs are not automatically turned into clickable links

warning: this URL is not a hyperlink
   --> src/async_impl/multipart.rs:419:5
    |
419 | /// https://url.spec.whatwg.org/#path-percent-encode-set
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use an automatic link instead: `<https://url.spec.whatwg.org/#path-percent-encode-set>`
    |
    = note: bare URLs are not automatically turned into clickable links

warning: this URL is not a hyperlink
   --> src/async_impl/multipart.rs:424:5
    |
424 | /// https://tools.ietf.org/html/rfc8187#section-3.2.1
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use an automatic link instead: `<https://tools.ietf.org/html/rfc8187#section-3.2.1>`
    |
    = note: bare URLs are not automatically turned into clickable links