Added Vector calculus under math section (#5238)

* Fix Contributing.md broken link

* Fixed broken links in free-programming-books-pt_BR.md

* Removed the broken link

There is no other link found, related to the book.

* Added vector calculus book

books/free-programming-books.md

@@ -592,6 +592,7 @@
 * [The Joy of Cryptography (2020)](https://web.engr.oregonstate.edu/~rosulekm/crypto/) - Mike Rosulek (PDF) (:construction: *in process*)
 * [Think Bayes: Bayesian Statistics Made Simple](http://www.greenteapress.com/thinkbayes/) - Allen B. Downey
 * [Think Stats: Probability and Statistics for Programmers](http://greenteapress.com/thinkstats/) - Allen B. Downey (using Python)
+* [Vector calculus](http://www.mecmath.net/calc3book.pdf) - Michael Corral (PDF)
 * [Yet Another Introductory Number Theory Textbook](https://www.poritz.net/jonathan/share/yaintt.pdf) - Jonathan A. Poritz (PDF)