Fixes: #6178 (#6179)

* Fixes: #6178

* Upate Julia Quantitative Economics link to be more stable

books/free-programming-books-langs.md

@@ -1324,7 +1324,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Julia Data Science](https://juliadatascience.io) - Jose Storopoli, Rik Huijzer, and Lazaro Alonso
 * [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)
+* [Quantitative Economics with Julia](https://julia.quantecon.org) - Jesse Perla, Thomas J. Sargent and John Stachurski (HTML, [PDF](https://web.archive.org/web/20210713122108/https://julia.quantecon.org/_downloads/pdf/quantitative_economics_with_julia.pdf))
 * [The Julia Express](http://bogumilkaminski.pl/files/julia_express.pdf) - Bogumił Kamiński (PDF)
 * [Think Julia](https://benlauwens.github.io/ThinkJulia.jl/latest/book.html) - Ben Lauwens and Allen Downey (GitBook)