How can I add “last updated” in plugin descripton?

Use this code instead:

function range_plu_plugin_meta( $plugin_meta, $plugin_file ) {
    list( $slug ) = explode( "https://wordpress.stackexchange.com/", $plugin_file );


    $slug_hash = md5( $slug );
    $last_updated = get_transient( "range_plu_{$slug_hash}" );
    if ( false === $last_updated ) {
        $last_updated = range_plu_get_last_updated( $slug );
        set_transient( "range_plu_{$slug_hash}", $last_updated, 86400 );
    }

    if ( $last_updated ) {
        $last_updated = strtotime( $last_updated );
        $last_updated = human_time_diff( $last_updated, current_time( 'timestamp' ) ) . ' ' . __( 'ago' );
        $plugin_meta['last_updated'] = 'Last Updated: ' . esc_html( $last_updated );
    }

    return $plugin_meta;
}