Add back calculus by gilbert strang (#11249)

* Add back Calculus by Gilbert Strang

Add working link pointing to the updated 3rd edition of the book.

* Add format

* Change the link to point to the textbook page.

Author: vshenoy

books/free-programming-books-subjects.md

@@ -480,6 +480,7 @@ Books that cover a specific programming language can be found in the [BY PROGRAM
 * [Bayesian Methods for Hackers](https://github.com/CamDavidsonPilon/Probabilistic-Programming-and-Bayesian-Methods-for-Hackers) - Cameron Davidson-Pilon
 * [Beginning and Intermediate Algebra](http://www.wallace.ccfaculty.org/book/book.html) - Tyler Wallace
 * [Book of Proof](https://www.people.vcu.edu/~rhammack/BookOfProof/) - Richard Hammack [(PDF)](https://www.people.vcu.edu/~rhammack/BookOfProof/Main.pdf)
+* [Calculus](https://ocw.mit.edu/courses/res-18-001-calculus-fall-2023/pages/textbook/) - Gilbert Strang (PDF)
 * [Calculus I](https://resolver.caltech.edu/CaltechBOOK:1985.001) - Jerrold E. Marsden, Alan Weinstein
 * [Calculus in Context](http://www.math.smith.edu/~callahan/intromine.html) - James Callahan
 * [Calculus Made Easy](https://www.gutenberg.org/ebooks/33283) - Silvanus P. Thompson (PDF)