Apply "Getting Real" book redirect (#5462)

Apply 301 Redirect before resource link brokes.

From: http://gettingreal.37signals.com
To: https://basecamp.com/books/getting-real

Notified at EbookFoundation/free-programming-books#5233 by @Astrodevil

books/free-programming-books.md

@@ -675,7 +675,7 @@
 * [Game AI Pro](http://www.gameaipro.com) - Steve Rabin
 * [Game Design with AGS](https://ensadi.github.io/AGSBook/) - Dave Ensminger, A. G. Madi
 * [Game Programming Patterns](http://gameprogrammingpatterns.com) - Bob Nystrom
-* [Getting Real](http://gettingreal.37signals.com)
+* [Getting Real](https://basecamp.com/books/getting-real) - Basecamp, 37signals ([HTML](https://basecamp.com/gettingreal), [PDF](https://basecamp.com/gettingreal/getting-real.pdf))
 * [Google Maps API Succinctly](https://www.syncfusion.com/ebooks/google_maps_api_succinctly) - Mark Lewin
 * [Hacknot: Essays on Software Development](http://www.lulu.com/shop/ed-johnson/hacknot-essays-on-software-development/ebook/product-17544641.html) - Ed Johnson
 * [Hello SDL](https://lazyfoo.net/tutorials/SDL) - Lazy Foo' Productions