Update README.md - Custom segment format fix

[ChrisToxz]

Custom segment format returns array, needs to be a string.

Default generator in HLSExporter.php

/**
     * Returns a default generator if none is set.
     *
     * @return callable
     */
    private function getSegmentFilenameGenerator(): callable
    {
        return $this->segmentFilenameGenerator ?: function ($name, $format, $key, $segments, $playlist) {
            $segments("{$name}_{$key}_{$format->getKiloBitrate()}_%05d.ts");
            $playlist("{$name}_{$key}_{$format->getKiloBitrate()}.m3u8");
        };
    }

[pascalbaljet]

Thanks @ChrisToxz!