fix wording

Author: vladak

intro.tex

@@ -249,7 +249,7 @@
 \begin{itemize}
 \item The first document is \emph{IEEE Std POSIX1003.1-1988}, formerly simply
 referred to as POSIX, then referenced as \emph{POSIX.1}, because by POSIX it is
-currently meant a set of related standards. POSIX.1 in back then contained
+currently meant a set of related standards. POSIX.1 back then contained
 programming API, i.e. work with processes, signals, files, timers etc.
 It was accepted by the ISO organization (\emph{ISO 9945-1:1990}) With small
 changes and is referred to as POSIX1990. IEEE reference is