Fix typo in Threading and Tasks doc

Change-Id: Ib3b297674c11fd9d3ed71a775b681ba6d2f65fab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2041873
Auto-Submit: Oliver Li <olivierli@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Oliver Li <olivierli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740283}

docs/threading_and_tasks.md

@@ -337,7 +337,7 @@ posting order.
 ### Posting to the Main Thread or to the IO Thread in the Browser Process
 
 To post tasks to the main thread or to the IO thread, use
-`content::GetUIThreadTaskRunner({})` or `content::GetUIThreadTaskRunner({})`
+`content::GetUIThreadTaskRunner({})` or `content::GetIOThreadTaskRunner({})`
 from
 [`content/public/browser/browser_thread.h`](https://cs.chromium.org/chromium/src/content/public/browser/browser_thread.h)