specify encoding to ensure Python reads file as UTF-8

executing `python whisper_fastapi_online_server.py --host 0.0.0.0 --port 8000` resulted in error on my setup for me:

```
whisper_streaming_web\whisper_fastapi_online_server.py, line 47, in <module>
html = f.read()
^^^^^^^^
File "C:\Python312\Lib\encodings\cp1252.py", line 23, in decode
return codecs.charmap_decode(input,self.errors,decoding_table)[0]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
UnicodeDecodeError: 'charmap' codec can't decode byte 0x8f in position 1818: character maps to <undefined>
```

On Windows, Python defaults to the `cp1252` encoding, which may not match the encoding of the file being read.
Files containing special characters, non-ASCII text, or saved with UTF-8 encoding can trigger this error when read without specifying the correct encoding.

whisper_fastapi_online_server.py

@@ -43,7 +43,7 @@ args = parser.parse_args()
 asr, tokenizer = backend_factory(args)
 
 # Load demo HTML for the root endpoint
-with open("src/live_transcription.html", "r") as f:
+with open("src/live_transcription.html", "r", encoding="utf-8") as f:
     html = f.read()