fix wording

Author: Vladimir Kotal

intro.tex

@@ -2087,7 +2087,7 @@
 \item Using numbers for options (ala \texttt{-4} and \texttt{-6} for some
 \texttt{ping} implementations) is old fashioned;
 reportedly SUSv3 mentions that.
-\item Watch out for GNU command line utilities and its strange permutations
+\item Watch out for GNU command line utilities and their strange permutations
 of options and arguments.
 \item The \texttt{-W} option should be reserved for vendor options, i.e. for
 non-portable extensions.