doc: using triple backticks instead of <pre> blocks

While <pre> HTML tag blocks are allowed in both commonmark specification
and commonmark-java, for some reason,
webrtc.googlesource.com using gitiles doesn't render that block. [1]
It's probably because of the stricter conditions of the gitiles HTML
extension. [2]
So use a much more portable code block syntax (triple backticks).

[1] https://webrtc.googlesource.com/src/+/5900ba0ee8f3f9cef3b29becbb4335b8f440d57d/api/g3doc/threading_design.md
[2] https://gerrit.googlesource.com/gitiles/+/f65ff3b7bfc36f8426aa0199220b111e14ff92ee/java/com/google/gitiles/doc/GitilesHtmlExtension.java#32

Bug: None
Change-Id: Ie83bbb7e26dec5225cd79b926b97529e33a37149
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/225360
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34433}
diff --git a/api/g3doc/threading_design.md b/api/g3doc/threading_design.md
index 868c433..20c3539 100644
--- a/api/g3doc/threading_design.md
+++ b/api/g3doc/threading_design.md
@@ -37,7 +37,7 @@
 At the moment, the API does not give any guarantee on which thread* the
 callbacks and events are called on. So it's best to write all callback
 and event handlers like this (pseudocode):
-<pre>
+```
 void ObserverClass::Handler(event) {
   if (!called_on_client_thread()) {
     dispatch_to_client_thread(bind(handler(event)));
@@ -45,7 +45,7 @@
   }
   // Process event, we're now on the right thread
 }
-</pre>
+```
 In the future, the implementation may change to always call the callbacks
 and event handlers on the client thread.