fix typo

Author: Vladimir Kotal

threads.tex

@@ -212,7 +212,7 @@
 
 \begin{itemize}
 \item This is a trivial example.  The process (main thread) creates two more
-threads and waits for them to finish.  This process thus have 3 threads in
+threads and waits for them to finish.  This process thus has 3 threads in
 total.
 \end{itemize}