fix spellcheck

Author: Vladimir Kotal

threads.tex

@@ -1284,7 +1284,7 @@
 be wrapped under \texttt{\_\_STDC\_NO\_ATOMICS\_\_} negative define.
 \item A good description of the C11 atomic API can be found on
 \url{http://en.cppreference.com/w/c/atomic}. Some systems have the
-stdatomic(3) man page.
+\texttt{stdatomic(3)} man page.
 \end{itemize}