Line-headings fails for `code` beginning lines

Line-headings (`====` or `----` line following heading text)
does not work for the lines that begin with `` `code` ``.

```text
hogefuga (OK)
====

hogefuga (OK)
----

`hoge` fuga (NG)
====

`hoge` fuga (NG)
----
```

![markdown](https://github.com/preservim/vim-markdown/assets/31273423/3aba73ce-ccc8-4f4f-a625-41c46cef15ec)