Update docs/helpers.md

Co-authored-by: Luffy <lufei@docsifyjs.org>

Author: Koooooo-7

docs/helpers.md

@@ -92,6 +92,9 @@ You will get `<a href="/demo/">link</a>`html. Do not worry, you can still set th
 ### Class names
 
 ```md
+![logo](https://docsify.js.org/_media/icon.svg ':class=someCssClass')
+
+<!-- Multiple class names -->
 ![logo](https://docsify.js.org/_media/icon.svg ':class=someCssClass :class=anotherCssClass')
 ```