Fix broken link (EbookFoundation#5284)

Fix broken link (Julia book). Now it work

Author: IgorLutiy

books/free-programming-books.md

@@ -1956,7 +1956,7 @@ Kerridge (PDF) (email address *requested*, not required)
 
 * [Introducing Julia](https://en.wikibooks.org/wiki/Introducing_Julia) - Wikibooks
 * [Julia by Example](http://samuelcolvin.github.io/JuliaByExample) - Samuel Colvin (GitHub repo)
-* [Julia language: a concise tutorial](https://www.gitbook.com/book/sylvaticus/julia-language-a-concise-tutorial) - Antonello Lobianco (GitBook)
+* [Julia language: a concise tutorial](https://syl1.gitbook.io/julia-language-a-concise-tutorial) - Antonello Lobianco (GitBook)
 * [Learn Julia in Y minutes](https://learnxinyminutes.com/docs/julia) - Leah Hanson
 * [Quantitative Economics with Julia](https://julia.quantecon.org/_downloads/pdf/quantitative_economics_with_julia.pdf) - Jesse Perla, Thomas J. Sargent and John Stachurski (PDF)
 * [The Julia Express](http://bogumilkaminski.pl/files/julia_express.pdf) - Bogumił Kamiński (PDF)