Update app.py

app.py

@@ -407,7 +407,7 @@ class WhisperTranscriber:
 
         output_files = []
         output_files.append(self.__create_file(srt, output_dir, source_name + "-subs.srt"));
-        output_files.append(self.__create_file(vtt, output_dir, source_name + "-subs.vtt"));
+        output_files.append(self.__create_file(vtt, output_dir, source_name + "-subs.txt"));
         output_files.append(self.__create_file(text, output_dir, source_name + "-transcript.txt"));
         output_files.append(self.__create_file(json_result, output_dir, source_name + "-result.json"));