[Tests] Remove deprecation messages in tests

Author: lindyhopchris

tests/database/factories/PostFactory.php

@@ -39,7 +39,7 @@ public function definition()
     {
         return [
             'content' => $this->faker->paragraphs(3, true),
-            'slug' => $this->faker->unique()->slug,
+            'slug' => $this->faker->unique()->slug(),
             'title' => $this->faker->words(5, true),
         ];
     }