Update C++ comment format examples

Update the example to match the new recommended style (use backticks).

Change-Id: I635c1dc0fabf1581f04f11c4ac9a98fda6edebdf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2277079
Auto-Submit: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784163}

styleguide/c++/c++-dos-and-donts.md

@@ -284,8 +284,8 @@ cross-references for that symbol.
   at least provide a starting point for asking about an issue.
 
 ```cpp
-// FooImpl implements the FooBase class.
-// FooFunction() modifies |foo_member_|.
+// `FooImpl` implements the `FooBase` class.
+// `FooFunction()` modifies `foo_member_`.
 // TODO(crbug.com/1): Rename things to something more descriptive than "foo".
 ```