Fix incorrect comment

Review URL: https://codereview.webrtc.org/1524663004

Cr-Commit-Position: refs/heads/master@{#11036}

webrtc/modules/audio_coding/codecs/audio_encoder.h

@@ -125,7 +125,7 @@ class AudioEncoder {
   // Tells the encoder about the highest sample rate the decoder is expected to
   // use when decoding the bitstream. The encoder would typically use this
   // information to adjust the quality of the encoding. The default
-  // implementation just returns true.
+  // implementation does nothing.
   virtual void SetMaxPlaybackRate(int frequency_hz);
 
   // Tells the encoder what the projected packet loss rate is. The rate is in