update docs example

Author: browner12

readme.md

@@ -374,7 +374,7 @@ try {
 If you would like to use a modifier with a value not generated by Faker, use the `passthrough()` method. `passthrough()` simply returns whatever value it was given.
 
 ```php
-$faker->optional()->passthrough('my custom value');
+$faker->optional()->passthrough(mt_rand(5, 15));
 ```
 
 ## Localization