radicle: fix word in RefsAt description — heartwood

Signed-off-by: Lars Wirzenius liw@liw.fi

 /// The content-addressable information required to load a remote's
 /// `rad/sigrefs`.
 ///
 /// This can be used to [`RefsAt::load`] a [`SignedRefsAt`].
 /// Use [`RefsAt::load`] to produce a [`SignedRefsAt`].
 ///
 /// It can also be used for communicating announcements of updates
 /// `RefsAt` can also be used for communicating announcements of updates
 /// references to other nodes.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]