Update free-programming-books.md

Fixed typo.
Add C book "Using C on the UNIX System"

By David A. Curry. Made available for personal use by the author.
Merge pull request #804 from chastai/add-book

Add C book "Using C on the UNIX System"
[es] Adding C book
Merge pull request #805 from esparta/master

[es] Adding C book
added pt-BR version of Official Python Tutorial
Merge pull request #806 from luzfcb/patch-1

added pt-BR version of Official Python Tutorial
added Windows Phone 8 Development Succinctly

added free ebook "Windows Phone 8 Development Succinctly" from syncfusion.

link: http://www.syncfusion.com/resources/techportal/ebooks/windowsphone8
added author and file type
Merge pull request #807 from Misterhex/master

added Windows Phone 8 Development Succinctly
[es] New section (& book): Erlang
Adding Code Guide to HTML/CSS section.
Add a single haskell book
[es] Adding Erlang book
append two parallel programming books
Merge pull request #808 from HungMingWu/master

Append two parallel books
Polish translation of PHP: The right way.
Polish translation of Pro Git book
Merge pull request #809 from piotrl/master

Polish translations of PHP and Git books.
Fixed Django Anchor in Index