style: use Self to avoid unnecessary repetition (#3396)

axum-extra/src/response/multiple.rs

@@ -25,7 +25,7 @@ impl MultipartForm {
     /// let form = MultipartForm::with_parts(parts);
     /// ```
     pub fn with_parts(parts: Vec<Part>) -> Self {
-        MultipartForm { parts }
+        Self { parts }
     }
 }