fix wording for pthread_setspecific()

Author: vladak

threads.tex

@@ -523,7 +523,7 @@
 \\\>const void *\emph{value});}
 ]]])
 \begin{itemize}
-\item sets pointer \emph{value} to \emph{key}
+\item binds pointer \emph{value} to previously created \emph{key}
 \end{itemize}
 ifdef([[[NOSPELLCHECK]]], [[[
 \texttt{void *\funnm{pthread\_getspecific}(pthread\_key\_t \emph{key});}