fix typos

Author: Vladimir Kotal

threads.tex

@@ -581,7 +581,7 @@
 \end{itemize}
 \texttt{void \funnm{pthread\_cleanup\_pop}(int \emph{execute});}
 \begin{itemize}
-\item removed a routing from the top of the stack.  Will call the routine if
+\item remove a routine from the top of the stack.  Will call the routine if
 \emph{execute} is non-zero
 \end{itemize}
 \end{slide}