chore: add mdown keyword in _tools/remark/plugins/remark-lint-expected-html-sections

Adds the missing `mdown` keyword to align with the established
`@stdlib/_tools/remark/plugins/*` keyword convention (12 of 14 sibling
packages list `mdown` alongside `markdown` / `md`).

Author: claude

lib/node_modules/@stdlib/_tools/remark/plugins/remark-lint-expected-html-sections/package.json

@@ -41,6 +41,7 @@
     "linter",
     "markdown",
     "md",
+    "mdown",
     "remark",
     "remark-plugin",
     "plugin",