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

Bug: webrtc:12338
Change-Id: I85bff694dd2ead83c939c4d1945eff82e1296001
No-Presubmit: True
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/227161
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34690}

modules/audio_processing/gain_controller2_unittest.cc

@@ -27,7 +27,7 @@ namespace test {
 namespace {
 
 void SetAudioBufferSamples(float value, AudioBuffer* ab) {
-  // Sets all the samples in |ab| to |value|.
+  // Sets all the samples in `ab` to `value`.
   for (size_t k = 0; k < ab->num_channels(); ++k) {
     std::fill(ab->channels()[k], ab->channels()[k] + ab->num_frames(), value);
   }