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

Bug: webrtc:12338
Change-Id: I2a33903a79194bb092a17ea1e1505bf2a3377d8b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/227027
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34558}

test/time_controller/external_time_controller.h

@@ -53,7 +53,7 @@ class ExternalTimeController : public TimeController, public TaskQueueFactory {
   class TaskQueueWrapper;
 
   // Executes any tasks scheduled at or before the current time.  May call
-  // |ScheduleNext| to schedule the next call to |Run|.
+  // `ScheduleNext` to schedule the next call to `Run`.
   void Run();
 
   void UpdateTime();