docs: fix typo

Author: ulivz

Diff for: docs/guide/using-vue.md

@@ -126,7 +126,7 @@ Inside any markdown file you can then directly use the components (names are inf
 Make sure a custom component's name either contains a hyphen or is in PascalCase. Otherwise it will be treated as an inline element and wrapped inside a `<p>` tag, which will lead to hydration mismatch because `<p>` does not allow block elements to be placed inside it.
 :::
 
-### Using Pre-processors
+## Using Pre-processors
 
 VuePress has built-in webpack config for the following pre-processors: `sass`, `scss`, `less`, `stylus` and `pug`. All you need to do is installing the corresponding dependencies. For example, to enable `sass`, install the following in your project: