add releases link

Author: Vladimir Kotal

intro.tex

@@ -40,7 +40,9 @@
 BY-NC-SA 3.0} license and is always a work in progress, see the history on
 GitHub:\\
 \url{https://github.com/devnull-cz/unix-linux-prog-in-c}
-\item To download the latest version, go to the \emph{releases} tab on GitHub.
+\item To download the latest version, go to the
+\href{https://github.com/devnull-cz/unix-linux-prog-in-c/releases}{releases}
+on GitHub.
 \item Source code referenced from this material is published in
 \href{http://creativecommons.org/licenses/publicdomain/}{Public Domain} unless
 specified otherwise in the files.