metadata: Fix some rustdoc::bare_urls warnings

Fix some `rustdoc::bare_urls`` warnings, due to a `rustdoc` lint that
was recently introduced in stable Rust [1]. By default, bare URLs
trigger a warning and the developer must decide if they want to render
them as links, or as literal values.

In our case, we have two URLs in our footnotes that we want to render as
hyperlinks, so we enclose them in <...>.

[1]: rust-lang/rust#81764

CHANGELOG.md

@@ -7,6 +7,12 @@ Versioning].
 
 ## [Unreleased]
 
+### Fixed
+
+- Fix a `rustdoc::bare_urls`` warning for some links that we used in our
+  footnotes and did not have any style indication, by formatting them as
+  hyperliks.
+
 ## [0.3.1] - 2021-04-20
 
 ### Fixed

src/metadata.rs

@@ -90,7 +90,7 @@ use protobuf::Message;
 /// recommended size by IETF [^ietf], and because the Ring library does not
 /// accept any other size.
 ///
-/// [^ietf]: From https://tools.ietf.org/html/rfc5084#section-3.2:
+/// [^ietf]: From <https://tools.ietf.org/html/rfc5084#section-3.2>:
 ///          _A length of 12 octets is RECOMMENDED._
 pub const RING_NONCE_SIZE: usize = 12;
 
@@ -129,7 +129,7 @@ pub const PBKDF2_DEFAULT_ITERATIONS: usize = 100000;
 /// because it's generally better if the output of the HMAC function matches
 /// the length of the desired key [^pbkdf2-design-flaw].
 ///
-/// [^pbkdf2-design-flaw]: https://www.chosenplaintext.ca/2015/10/08/pbkdf2-design-flaw.html
+/// [^pbkdf2-design-flaw]: <https://www.chosenplaintext.ca/2015/10/08/pbkdf2-design-flaw.html>
 pub const PBKDF2_DEFAULT_HASH_FN: HashFunction = HashFunction::SHA256;
 
 /// The hash functions that this library supports.