fix(docs): Inconsistent usage of ::: warning tag (quasarframework#5198)

Author: webnoob

docs/src/pages/vue-components/tree.md

@@ -28,7 +28,7 @@ Notice (in the example below) the custom header and body slots.
 
 <doc-example title="Customizing nodes" file="QTree/SlotsCustomized" />
 
-:::warning
+::: warning
 Clicking or pressing `SPACE` or `ENTER` on the custom header selects the tree item (and the custom header is blurred).
 
 If you don't want this to happen just wrap the content of the custom header in a `<div @click.stop @keyup.13.32.stop>`.