Commit stray Rust change that keeps popping up (rust/src/canonical_json.rs) (#19763)

(introduced in https://github.com/element-hq/synapse/pull/19739)

Seems like some automatic change from `poetry run ./scripts-dev/lint.sh`

changelog.d/19763.misc

@@ -0,0 +1 @@
+Lint and format `rust/src/canonical_json.rs`.

rust/src/canonical_json.rs

@@ -824,7 +824,7 @@ mod tests {
 
         // Serialize the keys in the reverse order.
         for (c, _) in ascii_order.iter().rev() {
-            map_serializer.serialize_entry(c.into(), &1).unwrap();
+            map_serializer.serialize_entry(c, &1).unwrap();
         }
         SerializeMap::end(map_serializer).unwrap();