WordPress custom post type slug rewriting

        $labels = array(
            'name'               => _x( 'Portfolio' , 'post type general name', 'maxive' ),
            'singular_name'      => _x( 'Portfolio', 'post type singular name', 'maxive' ),
            'menu_name'          => _x( 'Portfolio' , 'admin menu', 'maxive' ),
            'name_admin_bar'     => _x( 'Portfolio', 'add new on admin bar', 'maxive' ),
            'add_new'            => __( 'Add New Portfolio', 'maxive' ),
            'add_new_item'       => __( 'Add New Portfolio', 'maxive' ),
            'new_item'           => __( 'New Portfolio', 'maxive' ),
            'edit_item'          => __( 'Edit Portfolio', 'maxive' ),
            'view_item'          => __( 'View Portfolio', 'maxive' ),
            'all_items'          => __( 'All Portfolios', 'maxive' ),
            'search_items'       => __( 'Search Portfolios', 'maxive' ),
            'parent_item_colon'  => __( 'Parent :', 'maxive' ),
            'not_found'          => __( 'No Portfolios found.', 'maxive' ),
            'not_found_in_trash' => __( 'No Portfolios found in Trash.', 'maxive' )
        );

        $args = array(
            'labels'             => $labels,
            'description'        => __( 'Description.', 'maxive' ),
            'public'             => true,
            'publicly_queryable' => true,
            'show_ui'            => true,
            'show_in_menu'       => true,
            'query_var'          => true,
            'menu_icon'          => 'dashicons-businessman',
            'rewrite'            => array( 'slug' => 'Portfolio', 'with_front' => true, 'pages' => true, 'feeds' => true ),
            'capability_type'    => 'post',
            'has_archive'        => true,
            'hierarchical'       => true,
            'menu_position'      => null,
            'menu_icon'          => 'dashicons-admin-appearance',
            'supports'           => array( 'title', 'editor', 'thumbnail', 'excerpt', 'page-attributes')
        );

        register_post_type('portfolio', $args);