[options] Fix thumbnail option group name (closes #29042)

youtube_dl/options.py

@@ -768,7 +768,7 @@ def parseOpts(overrideArguments=None):
         action='store_true', dest='rm_cachedir',
         help='Delete all filesystem cache files')
 
-    thumbnail = optparse.OptionGroup(parser, 'Thumbnail images')
+    thumbnail = optparse.OptionGroup(parser, 'Thumbnail Options')
     thumbnail.add_option(
         '--write-thumbnail',
         action='store_true', dest='writethumbnail', default=False,