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

Bug: webrtc:12338
Change-Id: I88cf10afa5fc810b95d2a585ab2e895dcc163b63
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/226953
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34575}

pc/ice_server_parsing_unittest.cc

@@ -23,7 +23,7 @@ namespace webrtc {
 class IceServerParsingTest : public ::testing::Test {
  public:
   // Convenience functions for parsing a single URL. Result is stored in
-  // |stun_servers_| and |turn_servers_|.
+  // `stun_servers_` and `turn_servers_`.
   bool ParseUrl(const std::string& url) {
     return ParseUrl(url, std::string(), std::string());
   }