Updated Lua reference manual version (#2968)

updated the reference manual to Lua's latest edition

free-programming-books.md

@@ -1733,7 +1733,7 @@ Kerridge (PDF) (email address *requested*, not required)
 
 ### Lua
 
-* [Lua 5.1 Reference Manual](http://www.lua.org/manual/5.1/)
+* [Lua 5.3 Reference Manual](http://www.lua.org/manual/5.3/)
 * [Lua Programming](https://en.wikibooks.org/wiki/Lua_Programming) - Wikibooks
 * [Programming in Lua (first edition)](https://www.lua.org/pil/contents.html)