radicle: fix `parse_ref_*` documentation — heartwood

The parse_ref and parse_ref_namespaced documentation used 4 space indentation for the reference names. Unfortunately, cargo test attempts to interpret this as code.

Use triple-backtick code fencing, with text and no_run, instead.

 /// e.g. in the reference below, the segment is
 /// `z6MkvUJtYD9dHDJfpevWRT98mzDDpdAtmUjwyDSkyqksUr7C`:
 ///
 ///     refs/namespaces/z6MkvUJtYD9dHDJfpevWRT98mzDDpdAtmUjwyDSkyqksUr7C/refs/heads/main
 /// ```text, no_run
 /// refs/namespaces/z6MkvUJtYD9dHDJfpevWRT98mzDDpdAtmUjwyDSkyqksUr7C/refs/heads/main
 /// ```
 ///
 /// The `T` can be specified when calling the function. For example, if you
 /// wanted to parse the namespace as a `PublicKey`, then you would the function

 /// e.g. in the reference below, the segment is
 /// `z6MkvUJtYD9dHDJfpevWRT98mzDDpdAtmUjwyDSkyqksUr7C`:
 ///
 ///     refs/namespaces/z6MkvUJtYD9dHDJfpevWRT98mzDDpdAtmUjwyDSkyqksUr7C/refs/heads/main
 /// ```text, no_run
 /// refs/namespaces/z6MkvUJtYD9dHDJfpevWRT98mzDDpdAtmUjwyDSkyqksUr7C/refs/heads/main
 /// ```
 ///
 /// The `T` can be specified when calling the function. For example, if you
 /// wanted to parse the namespace as a `PublicKey`, then you would the function