Update free-programming-books-langs.md

[Preston1208]

What does this PR do?

Add resource(s)

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Description

Added a free online book A Guide to Lua from Opensource.com, a beginner-friendly Lua programming guide available in PDF and EPUB.

Why is this valuable (or not)?

Lua is a popular lightweight scripting language, and this resource is great for beginners wanting to learn Lua fundamentals for free.

How do we know it's really free?

The resource is freely available for download on Opensource.com, a trusted community platform for open source resources.

For book lists, is it a book? For course lists, is it a course? etc.

It is a book.

Checklist:

• Search for duplicates.
• Include author(s) and platform where appropriate.
• Put lists in alphabetical order, correct spacing.
• Add needed indications (PDF, access notes, under construction).
• Used an informative name for this pull request.

Follow-up

Checked GitHub Actions for errors, no warnings found.

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[eshellman]

Suggested change

	* [A Guide to Lua](https://opensource.com/downloads/learn-lua) - Opensource.com (PDF, EPUB)
	* [A Guide to Lua](https://opensource.com/downloads/learn-lua) - Seth Kenlon (PDF, EPUB)

Please add author names where appropriate.