Update README.md (Double quotes missing)

Change 
    ``php artisan db:seed --class=Fbf\LaravelCategories\CategoriesTableBaseSeeder`` 
by 
``php artisan db:seed --class="Fbf\LaravelCategories\CategoriesTableBaseSeeder"`` 
otherwise beginners can get trouble
Merge pull request #2 from mercuryseries/patch-1

Update README.md (Double quotes missing)