[vimeo] 'ext' must be a string, not a tuple (fixes #8288)

There was an ',' at the end of the line.

youtube_dl/extractor/vimeo.py

@@ -430,7 +430,7 @@ class VimeoIE(VimeoBaseInfoExtractor):
                 if download_url and not source_file.get('is_cold') and not source_file.get('is_defrosting'):
                     source_name = source_file.get('public_name', 'Original')
                     if self._is_valid_url(download_url, video_id, '%s video' % source_name):
-                        ext = source_file.get('extension', determine_ext(download_url)).lower(),
+                        ext = source_file.get('extension', determine_ext(download_url)).lower()
                         formats.append({
                             'url': download_url,
                             'ext': ext,