Fix typo (#9023)

Both "an `App" and "a `App" appear in this documentation and most probably "an `App" is the correct version, so this PR is for consistency.

See #9022 

Sorry @taylorotwell , I thought I found them all, but I missed this one.

Author: peterhauke

authorization.md

@@ -675,7 +675,7 @@ Laravel includes a middleware that can authorize actions before the incoming req
         // The current user may update the post...
     })->middleware('can:update,post');
 
-In this example, we're passing the `can` middleware two arguments. The first is the name of the action we wish to authorize and the second is the route parameter we wish to pass to the policy method. In this case, since we are using [implicit model binding](/docs/{{version}}/routing#implicit-binding), a `App\Models\Post` model will be passed to the policy method. If the user is not authorized to perform the given action, an HTTP response with a 403 status code will be returned by the middleware.
+In this example, we're passing the `can` middleware two arguments. The first is the name of the action we wish to authorize and the second is the route parameter we wish to pass to the policy method. In this case, since we are using [implicit model binding](/docs/{{version}}/routing#implicit-binding), an `App\Models\Post` model will be passed to the policy method. If the user is not authorized to perform the given action, an HTTP response with a 403 status code will be returned by the middleware.
 
 For convenience, you may also attach the `can` middleware to your route using the `can` method: