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.h

@@ -41,7 +41,7 @@ size_t hex_decode(char* buffer,
 
 // hex_decode, assuming that there is a delimiter between every byte
 // pair.
-// |delimiter| == 0 means no delimiter
+// `delimiter` == 0 means no delimiter
 // If the buffer is too short or the data is invalid, we return 0.
 size_t hex_decode_with_delimiter(char* buffer,
                                  size_t buflen,