updated the bash reference (#5355)

* Update free-programming-books.md

* Update free-programming-books.md

books/free-programming-books.md

@@ -1091,6 +1091,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [BashGuide](http://mywiki.wooledge.org/BashGuide) - Maarten Billemont (HTML) [(PDF)](http://s.ntnu.no/bashguide.pdf)
 * [Conquering the Command Line](http://conqueringthecommandline.com/book/frontmatter) - Mark Bates (HTML)
 * [Getting Started with BASH](http://www.hypexr.org/bash_tutorial.php) (HTML)
+* [GNU Bash manual](https://www.gnu.org/software/bash/manual/bash.pdf) (PDF)
 * [Google Shell Style Guide](https://google.github.io/styleguide/shell.xml) - Paul Armstrong (HTML)
 * [Introduction to Bash Scripting](https://github.com/bobbyiliev/introduction-to-bash-scripting) - Bobby Iliev (Markdown, PDF)
 * [Introduction to the Command Line](https://launchschool.com/books/command_line) - Launch School (HTML)