fix wording

Author: Vladimir Kotal

threads.tex

@@ -593,7 +593,7 @@
 \item The \funnm{pthread\_cleanup\_push} and \funnm{pthread\_cleanup\_pop}
 functions provide sort of a bracketing around code block that might need a
 cleanup. In fact, more often than not these functions are implemented as macros
-that open and close code blocks. This enforces this usage pattern
+that open and close a code block. This enforces this usage pattern
 (and causes trouble with \texttt{goto} statements).
 \item Run a C preprocessor on \example{pthreads/pthread-cleanup.c} and see how
 this is done.