fix typo

Author: Vladimir Kotal

intro.tex

@@ -2118,7 +2118,7 @@
 \item when all options are processed, it returns -1 and the \texttt{optind} 
 variable contains the index of first unprocessed argument of the program.
 \item possible options are passed in the \texttt{optstring} string.
-If an options has mandatory argument, its character is followed by '\texttt{:}'
+If an option has mandatory argument, its character is followed by '\texttt{:}'
 in the string.
 \item on error (unknown option, argument missing), the function returns
 '\texttt{?}', saves the option character into the \texttt{optopt} variable.