Allow Single Quantity in Cart Woocommerce

functions.php

add_filter ( 'woocommerce_before_cart' , 'allow_single_quantity_in_cart' );
   function allow_single_quantity_in_cart() {
           global $woocommerce;

           $cart_contents  =  $woocommerce->cart->get_cart();
           $keys           =  array_keys ( $cart_contents );

           foreach ( $keys as $key ) {
                   $woocommerce->cart->set_quantity ( $key, 1, true );
           }
   }