Add sample config for HEIC image previews

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

config/config.sample.php

@@ -952,6 +952,7 @@ $CONFIG = array(
  *
  *  - OC\Preview\BMP
  *  - OC\Preview\GIF
+ *  - OC\Preview\HEIC
  *  - OC\Preview\JPEG
  *  - OC\Preview\MarkDown
  *  - OC\Preview\MP3
@@ -963,6 +964,7 @@ $CONFIG = array(
 	'OC\Preview\PNG',
 	'OC\Preview\JPEG',
 	'OC\Preview\GIF',
+	'OC\Preview\HEIC',
 	'OC\Preview\BMP',
 	'OC\Preview\XBitmap',
 	'OC\Preview\MP3',