Fixed Essential Mathematics PDF Broken Link

[stmapman7]

I fixed the broken link for "The Handbook of Essential Mathematics" PDF.

What does this PR do?

Improve Repo

Fixes #118 for the broken link to The Handbook of Essential Mathematics.

For resources

Description

Why is this valuable (or not)

Users may now visit the updated PDF link.

How do we know it's really free?

I can download it directly as a PDF file with no issues.

For book lists, is it a book?

Yes, this is a book.

Checklist:

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

[eshellman]

Thanks!