fix: update angular material link (EbookFoundation#9212)

- angular 'guide' has been replaced with a collection of guides
- link updated to suit

Author: kevinpearce

books/free-programming-books-langs.md

@@ -2526,7 +2526,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Angular 2 Style Guide](https://github.com/johnpapa/angular-styleguide/blob/master/a2/README.md) - John Papa (HTML)
 * [Angular 2+ Notes for Professionals](https://goalkicker.com/Angular2Book/) - Compiled from StackOverflow documentation ([PDF](https://goalkicker.com/Angular2Book/Angular2NotesForProfessionals.pdf))
 * [Angular Docs](https://angular.io/docs) (HTML)
-* [Angular Material](https://material.angular.io/guide) (HTML)
+* [Angular Material](https://material.angular.io/guides) (HTML)
 * [Angular Tutorial](https://angular.io/tutorial) (HTML)
 * [Build a Full-Stack Web Application Using Angular & Firebase](https://www.c-sharpcorner.com/ebooks/build-a-full-stack-web-application-using-angular-and-firebase) - Ankit Sharma (PDF, [:package: code samples](https://github.com/AnkitSharma-007/blogging-app-with-Angular-CloudFirestore))