[mantine.dev] Fix headers structure in use-form documentation

apps/mantine.dev/src/pages/form/use-form.mdx

@@ -200,7 +200,7 @@ function Demo() {
 }
 ```
 
-## onSubmitPreventDefault option
+### onSubmitPreventDefault option
 
 By default, `event.preventDefault()` is called on the form `onSubmit` handler.
 If you want to change this behavior, you can pass `onSubmitPreventDefault` option