FIX compatibility with Laravel 6.8 (#307)

Remove -s option for searchable model generation to fix compatibility with laravel 6.8

Author: julienarcin

Diff for: src/Console/SearchableModelMakeCommand.php

@@ -41,7 +41,7 @@ protected function getOptions()
 
         $options[] = [
             'search-rule',
-            's',
+            null,
             InputOption::VALUE_REQUIRED,
             'Specify the search rule for the model. It\'ll be created if doesn\'t exist.',
         ];