Fix stylesheet HTML snippet for external renderers documentation (go-gitea#22435)

The documentation is missing the rel attribute. Neither Firefox nor
Chrome did use the linked file as CSS if rel="stylesheet" is not set.

The problem is described in issue go-gitea#22434.

Co-authored-by: silverwind <me@silverwind.io>

Author: sbannier

docs/content/doc/advanced/external-renderers.en-us.md

@@ -192,5 +192,5 @@ And so you could write some CSS:
 Add your stylesheet to your custom directory e.g `custom/public/css/my-style-XXXXX.css` and import it using a custom header file `custom/templates/custom/header.tmpl`:
 
 ```html
-<link type="text/css" href="{{AppSubUrl}}/assets/css/my-style-XXXXX.css" />
+<link rel="stylesheet" href="{{AppSubUrl}}/assets/css/my-style-XXXXX.css" />
 ```