Update config/config.sample.php

add better description

Co-authored-by: Joas Schilling <213943+nickvergessen@users.noreply.github.com>
Signed-off-by: William <william.hak57@gmail.com>

config/config.sample.php

@@ -1194,7 +1194,7 @@ $CONFIG = [
 /**
  * custom path for ffmpeg binary
  * 
- * Defaults to ``''`` (empty string)
+ * Defaults to ``null`` and falls back to searching ``avconv`` and ``ffmpeg`` in the configured ``PATH`` environment
  */
 'preview_ffmpeg_path' => '/usr/bin/ffmpeg',