Link to Programming in Lua (#2569)

the Lua Programming Gems book is not freely available (apart from the second chapter and notes for some other chapters), and is not a general guide to the language; the first edition of Programming in Lua, however, *is* freely available, and *is* a guide for learning the language in general.

free-programming-books.md

@@ -1537,7 +1537,7 @@ For resources on Angular, Backbone, D3, Dojo, Ember, Express, jQuery, Knockout,
 ### Lua
 
 * [Lua 5.1 Reference Manual](http://www.lua.org/manual/5.1/)
-* [Programming Gems](http://www.lua.org/gems/)
+* [Programming in Lua (first edition)](https://www.lua.org/pil/contents.html)
 * [Wikibook](https://en.wikibooks.org/wiki/Lua_Programming)