fix typo

Author: Vladimir Kotal

threads.tex

@@ -1161,7 +1161,7 @@
 \texttt{int \funnm{pthread\_rwlock\_destroy}(pthread\_rwlock\_t *\emph{l});}
 ]]])
 \begin{itemize}
-\item destroys the mutex
+\item destroys the lock
 \end{itemize}
 ifdef([[[NOSPELLCHECK]]], [[[
 \texttt{int \funnm{pthread\_rwlock\_rdlock}(pthread\_rwlock\_t *\emph{l});}\\