clarify that Icon::register() should go in the service providers boot method

Author: duncanmcclean

content/collections/pages/5-to-6.md

@@ -282,7 +282,11 @@ If you are using any `icon` fieldtypes with the `directory` option, you will nee
 ```php
 // AppServiceProvider.php
 use Statamic\Facades\Icon; // [tl! ++]
-Icon::register('heroicons', base_path('resources/heroicons')); // [tl! ++]
+
+public function boot(): void
+{
+    Icon::register('heroicons', base_path('resources/heroicons')); // [tl! ++]
+}
 ```
 
 ```yaml