Use backticks not vertical bars to denote variables in comments for /p2p

Bug: webrtc:12338
Change-Id: Ie047b750cdf7ea2efe7a4632d18d5ed719c5ea83
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/226952
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34570}

p2p/base/stun_request.cc

@@ -106,7 +106,7 @@ void StunRequestManager::Clear() {
 
   for (uint32_t i = 0; i < requests.size(); ++i) {
     // StunRequest destructor calls Remove() which deletes requests
-    // from |requests_|.
+    // from `requests_`.
     delete requests[i];
   }
 }