doc(sdk): Fix a markup.

Even if the visual output is the same, it's semantically better to have
`<a><code>…</code></a>` rather than `<code><a>…</a></code>` I believe.

crates/matrix-sdk/src/sliding_sync/sticky_parameters.rs

@@ -55,7 +55,7 @@ pub trait StickyData {
 ///
 /// Initially, the provided data is considered to be invalidated, so it's
 /// applied onto the request the first time it's sent. Any changes to the
-/// wrapped data happen via `[Self::data_mut]`, which invalidates the sticky
+/// wrapped data happen via [`Self::data_mut`], which invalidates the sticky
 /// parameters; they will be applied automatically to the next request.
 ///
 /// When applying sticky parameters, we will also remember the transaction id