Use backticks not vertical bars to denote variables in comments for /api

Bug: webrtc:12338
Change-Id: Ib97b2c3d64dbd895f261ffa76a2e885bd934a87f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/226940
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34554}
diff --git a/api/audio/audio_frame.cc b/api/audio/audio_frame.cc
index c6e5cf4..0c39d51 100644
--- a/api/audio/audio_frame.cc
+++ b/api/audio/audio_frame.cc
@@ -52,7 +52,7 @@
 }
 
 void AudioFrame::ResetWithoutMuting() {
-  // TODO(wu): Zero is a valid value for |timestamp_|. We should initialize
+  // TODO(wu): Zero is a valid value for `timestamp_`. We should initialize
   // to an invalid value, or add a new member to indicate invalidity.
   timestamp_ = 0;
   elapsed_time_ms_ = -1;