fix: remove extra colon

Author: 1egoman

livekit-rtc/livekit/rtc/audio_frame.py

@@ -66,7 +66,7 @@ def __init__(
         self._sample_rate = sample_rate
         self._num_channels = num_channels
         self._samples_per_channel = samples_per_channel
-        self._userdata: = {} if userdata is None else userdata
+        self._userdata = {} if userdata is None else userdata
 
     @staticmethod
     def create(sample_rate: int, num_channels: int, samples_per_channel: int) -> "AudioFrame":