Update free-programming-books-subjects.md (Added machine learning book)

[Adarsh2801-r]

Added the English book - Pattern Recognition and Machine Learning by Christopher M Bishop under Machine Learning subject

What does this PR do?

Add resource(s)

For resources

Description

English book under Machine learning subject - Pattern Recognition and Machine Learning by Christopher M Bishop

Why is this valuable (or not)?

A widely used book in academia

How do we know it's really free?

Added the link to pdf

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

It is a book

Checklist:

• Read our contributing guidelines.
• 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

• Check the status of GitHub Actions and resolve any reported warnings!

[Adarsh2801-r]

@eshellman @sohansai Could you please review this PR?

[eshellman]

Thanks, and welcome to free-programming-books!