doc: fix typo in Concurrency Patterns article

Fixes #3178.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/6506064

doc/articles/concurrency_patterns.html

@@ -64,7 +64,7 @@ could fail since no one is ready.
 </p>
 
 <p>
-This problem is a textbook of example of what is known as a
+This problem is a textbook example of what is known as a
 <a href="https://en.wikipedia.org/wiki/Race_condition">race condition</a>, but
 the fix is trivial. We just make sure to buffer the channel <code>ch</code> (by
 adding the buffer length as the second argument to <a href="/pkg/builtin/#make">make</a>),