Added basic Probability Theory to Mathematics Section in free-science-books.md

[tanaynayak]

What does this PR do?

Add Resource(s)

For resources

Description

Basic Probability Theory by Robert B Ash, from the department of Mathematics at Uni of Illinois

Why is this valuable (or not)

It's a great book to get started with Probability theory in Mathematics

How do we know it's really free?

It's freely available on the university of Illinois website

For book lists, is it a book?

No

Checklist:

• [Y] Not a duplicate
• [Y] Included author(s) if appropriate
• [Y] Lists are in alphabetical order
• [Y] Needed indications added (PDF, access notes, under construction)

[eshellman]

Thanks, and welcome to free-science-books!