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

Bug: webrtc:12338
Change-Id: Id3b5081cf73be31829d75d7ef34942c2259053da
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/227096
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34674}

modules/pacing/pacing_controller.cc

@@ -346,7 +346,7 @@ Timestamp PacingController::NextSendTime() const {
   // If probing is active, that always takes priority.
   if (prober_.is_probing()) {
     Timestamp probe_time = prober_.NextProbeTime(now);
-    // |probe_time| == PlusInfinity indicates no probe scheduled.
+    // `probe_time` == PlusInfinity indicates no probe scheduled.
     if (probe_time != Timestamp::PlusInfinity() && !probing_send_failure_) {
       return probe_time;
     }