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/transient/click_annotate.cc

@@ -26,7 +26,7 @@ using webrtc::TransientDetector;
 // Creates a send times array, one for each step.
 // Each block that contains a transient, has an infinite send time.
 // The resultant array is written to a DAT file
-// Returns -1 on error or |lost_packets| otherwise.
+// Returns -1 on error or `lost_packets` otherwise.
 int main(int argc, char* argv[]) {
   if (argc != 5) {
     printf("\n%s - Application to generate a RTP timing file.\n\n", argv[0]);