[docs]: Make SingleThreadTaskRunner example more explicit

This CL makes a SingleThreadTaskRunner example more explicit by using
the SingleThreadTaskRunner type instead of using the
SequencedTaskRunner base class.

Bug: N/A
Change-Id: I77fa2affacbda0f4847e4b808eab267f0977fb49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636668Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Dominic Farolino <domfarolino@gmail.com>
Cr-Commit-Position: refs/heads/master@{#665049}

docs/threading_and_tasks.md

@@ -369,7 +369,7 @@ be the main thread or the IO thread, post them to a `base::SingleThreadTaskRunne
 created by `base::CreateSingleThreadTaskRunnerWithTraits`.
 
 ```cpp
-scoped_refptr<SequencedTaskRunner> single_thread_task_runner =
+scoped_refptr<SingleThreadTaskRunner> single_thread_task_runner =
     base::CreateSingleThreadTaskRunnerWithTraits(...);
 
 // TaskB runs after TaskA completes. Both tasks run on the same thread.