Fixes: #6178 (#6179)

* Fixes: #6178

* Upate Julia Quantitative Economics link to be more stable

books/free-programming-books-langs.md

 ## BY PROGRAMMING LANGUAGE
 
-Originally, this list included a section called "Language Agnostic" for books about programming subjects not restricted to a specific programming language. 
+Originally, this list included a section called "Language Agnostic" for books about programming subjects not restricted to a specific programming language.
 That section got so big, we decided to split it into its own file, the [BY SUBJECT file](free-programming-books-subjects.md).
 
 
@@ -773,7 +773,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Git Pocket Guide](https://www.oreilly.com/library/view/git-pocket-guide/9781449327507) - Richard E. Silverman
 * [Git Reference](https://web.archive.org/web/20170602211147/http://gitref.org/)
 * [Git Succinctly, Syncfusion](https://www.syncfusion.com/resources/techportal/ebooks/git) (PDF, Kindle) (email address *requested*, not required)
-* [Git-Tutorial For-Beginners](https://product.hubspot.com/blog/git-and-github-tutorial-for-beginners) - HubSpot Product Team 
+* [Git-Tutorial For-Beginners](https://product.hubspot.com/blog/git-and-github-tutorial-for-beginners) - HubSpot Product Team
 * [Git Workflows](http://documentup.com/skwp/git-workflows-book)
 * [Happy Git and GitHub for the useR](https://happygitwithr.com) - Jenny Bryan
 * [How to Collaborate on Github](https://github.com/eonist/How-to-collaborate-on-github) - André J
@@ -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)