[8.x] Fix model factories

[bastien-phi]

Fixes some small mistakes :

• typo in docblock
• Factory::definition should return array accordingly to usage (https://github.com/laravel/framework/blob/master/src/Illuminate/Database/Eloquent/Factories/Factory.php#L318)
• $count parameter of Factory::count() should be nullable (used with null in createOne and makeOne)