Update valid-username.md

Author: milwad-dev

docs/1.x/valid-username.md

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