sylius grid 'sorting' doesnt work fix

make your needed field for sorting `sortable: true` in `fields:` definition, for example:

sylius_grid:
    grids:
        my_grid:
            driver:
                name: doctrine/orm
                options:
                    class: App\Document\Record
            sorting:
                datetime: desc
            fields:
                datetime:
                    type: datetime
                    sortable: true