Fixed the broken bengali link

[akshat-chd]

What does this PR do?

Fixes a broken link in the Bengali C++ section.
Fixes #12904

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Description

This PR fixes a 404 broken link in the C++ section of books/free-programming-books-bn.md. (Closes #12904).

It removes the dead academia.edu link for "সিপিপি পরিগণনা Computer Programming" and replaces it with a new, working resource for a Bengali C++ book.

Why is this valuable (or not)?

The original link was broken, leading to a 404 error. This PR replaces it with a valid, high-quality, and free resource for Bengali-speaking learners, improving the quality of the list.

How do we know it's really free?

The linked page (shikkhok.com) is a public blog post on an educational website that offers the book as a free PDF download.

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

Yes, it is a book in PDF format.

Checklist:

• Search for duplicates. (The new link is not a duplicate).
• Include author(s) and platform where appropriate. (Author not listed on new resource).
• 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!

[eshellman]

book is not available from the substitute link. let's just remove it.

[github-actions]

Linter failed, fix the error(s):

free-programming-books/books/free-programming-books-bn.md
25:1-27:141  warning  Incorrect number of blank lines between last section and next heading  blank-lines-1-0-2  remark-lint

[akshat-chd]

Done ,I have removed the link @eshellman