fix: fix typo in link (#8509)

Author: MltStephane

billing.md

@@ -1406,7 +1406,7 @@ If you would like to offer trial periods without collecting the user's payment m
     ]);
 
 > **Warning**  
-> Be sure to add a [date cast](/docs/{{version}}/eloquent-mutators##date-casting) for the `trial_ends_at` attribute within your billable model's class definition.
+> Be sure to add a [date cast](/docs/{{version}}/eloquent-mutators#date-casting) for the `trial_ends_at` attribute within your billable model's class definition.
 
 Cashier refers to this type of trial as a "generic trial", since it is not attached to any existing subscription. The `onTrial` method on the billable model instance will return `true` if the current date is not past the value of `trial_ends_at`: