Update free-programming-books.md (#2728)

* Update free-programming-books.md

C Elements of Style was published by M&T books in 1992. This book covers only the C language and is a bit out dated. However it still contains a lot of good advice.

* add author

free-programming-books.md

@@ -875,6 +875,7 @@ Kerridge (PDF) (Just fill the fields with any values)
 * [Beej's Guide to C Programming](http://beej.us/guide/bgc/) - B. Hall
 * [Beej's Guide to Network Programming - Using Internet Sockets](http://beej.us/guide/bgnet/) - B. Hall
 * [Build Your Own Lisp](http://www.buildyourownlisp.com)
+* [C Elements of Style](http://www.oualline.com/books.free/style/) - Steve Oualline
 * [C for Python Programmers - Carl Burch](http://www.toves.org/books/cpy/)
 * [C Programming](https://en.wikibooks.org/wiki/Programming%3AC) - Wikibooks
 * [C Programming Boot Camp - Paul Gribble](http://www.gribblelab.org/CBootCamp/)