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

Bug: webrtc:12338
Change-Id: I72fcb505a92f03b2ace7160ee33d555a977eddfd
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/226955
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34587}

rtc_base/string_encode.cc

@@ -49,7 +49,7 @@ size_t hex_encode_output_length(size_t srclen, char delimiter) {
 }
 
 // hex_encode shows the hex representation of binary data in ascii, with
-// |delimiter| between bytes, or none if |delimiter| == 0.
+// `delimiter` between bytes, or none if `delimiter` == 0.
 void hex_encode_with_delimiter(char* buffer,
                                const char* csource,
                                size_t srclen,