mention pull requests

Author: vladak

intro.tex

@@ -45,7 +45,7 @@
 \url{https://github.com/devnull-cz/unix-linux-prog-in-c-src}
 \item In case you find any errors either in the text or in the example programs,
 we would appreciate you letting us know. Especially do not hesitate to create new
-issues on \url{https://github.com/devnull-cz/unix-linux-prog-in-c/issues}.
+issues and pull requests on \url{https://github.com/devnull-cz/unix-linux-prog-in-c/issues}.
 \end{itemize}
 
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