Update free-programming-books.md

Fixed typo.
Fix the URL for the Introduction to Data Science book
Merge pull request #797 from ryanoreilly/master

Fix the URL for the Introduction to Data Science book
Update Windows 8 book; adding a incomplete v2
Merge pull request #798 from KilFer/patch-1

Update Windows 8 book; adding a incomplete v2
20,000
Add a bash book:  Conquering the Command Line
[es] New section (& book): Erlang
Merge pull request #799 from HungMingWu/master

Add a bash book:  Conquering the Command Line
Merge pull request #800 from esparta/master

[es] New section (& book): Erlang
Adding Code Guide to HTML/CSS section.
Merge pull request #801 from jbranchaud/adding-html-css-resource

Adding Code Guide to HTML/CSS section.
Add a single haskell book
Merge pull request #802 from vikt0rs/master

Add one more book about haskell
[es] Adding Erlang book
Merge pull request #803 from esparta/master

[es] Adding Erlang book
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