`%method-doc` generates invalid HTML

[kazk]

This can be reproduced in 48abed4 so it's not caused by Marked update.

Input:

```%method-doc
{
  "method": "open_files",
  "desc": "Loops through any files that are not opened and opens them",
  "args": {
    "files": {
      "type": "Array<File>",
      "desc": "An array of file objects that may need to be opened"
    },
    "ext": {
      "type": "String",
      "desc": "An optional extension that may be used as a filter to determine which files to open"
    }
  }
}
```

Outputs something like this:

<div class="block block--docs method-doc">
<h3 id="h3_0">
  <code>
    <dfn class="doc-class">
      <dfn class="doc-name doc-name--method">open_files</dfn>
  </code>(<code><dfn class="doc-name doc-name--name">files</dfn></code>, <code><dfn class="doc-name doc-name--name">ext</dfn></code>)</h3><p>Loops through any files that are not opened and opens them</p></dfn>

</code> is unexpected there because <dfn class="doc-class"> that was opened in <code> is not closed. There's </dfn> all the way at the end.

This happens because marked-extensions handles @@doc tokens by global replacing strings carelessly :(