Fix formatting in README

Signed-off-by: Spencer Comin <[email protected]>

README.mdx

@@ -165,6 +165,7 @@ formatting rules and how they appear in `[%expect]` and `[%expect_exact]` blocks
 
 <details>
 <summary>Expand examples</summary>
+
 <!-- $MDX file=./test/negative-tests/for-mdx/mdx_cases.ml.corrected.expected,part=matching -->
 ```ocaml
 let%expect_test "matching behavior --- no content" =