Resolve doc_markdown pedantic lint on regression test function

    warning: you should put bare URLs between `<`/`>` or make a proper Markdown link
        --> test_suite/tests/test_annotations.rs:2383:25
         |
    2383 | /// Regression test for #1904
         |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `<https://github.com/serde-rs/serde/issues/1904>`
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_markdown
         = note: `-W clippy::doc-markdown` implied by `-W clippy::pedantic`
         = help: to override `-W clippy::pedantic` add `#[allow(clippy::doc_markdown)]`

test_suite/tests/test_annotations.rs

@@ -2380,7 +2380,7 @@ fn test_partially_untagged_enum_desugared() {
     );
 }
 
-/// Regression test for https://github.com/serde-rs/serde/issues/1904
+// Regression test for https://github.com/serde-rs/serde/issues/1904
 #[test]
 fn test_enum_tuple_and_struct_with_flatten() {
     #[derive(Serialize, Deserialize, PartialEq, Debug)]