Update valid-slug.md

Author: milwad-dev

docs/1.x/valid-slug.md

@@ -1,6 +1,6 @@
 ## ValidSlug
 
-If you want to validate slug, you can use `ValidSlug` rule:
+If you want to validate slug, you can use the `ValidSlug` rule:
 
 ```php
 use Milwad\LaravelValidate\Rules\ValidSlug;