2639: fix Linear Algebra links, and add Introduction to Proofs book (#2642)

free-programming-books.md

@@ -425,13 +425,14 @@
 * [Graph Theory](http://compalg.inf.elte.hu/~tony/Oktatas/TDK/FINAL/)
 * [Introduction to Probability](http://www.dartmouth.edu/~chance/teaching_aids/books_articles/probability_book/book.html) - Charles M. Grinstead and J. Laurie Snell
 * [Introduction to Probability and Statistics Spring 2014](http://ocw.mit.edu/courses/mathematics/18-05-introduction-to-probability-and-statistics-spring-2014/)
+* [Introduction to Proofs](http://joshua.smcvt.edu/proofs/) - Jim Hefferon
 * [Introduction to Statistical Thought](http://people.math.umass.edu/~lavine/Book/book.html) - Michael Lavine
 * [Kalman and Bayesian Filters in Python](https://github.com/rlabbe/Kalman-and-Bayesian-Filters-in-Python)
 * [Knapsack Problems - Algorithms and Computer Implementations](http://www.or.deis.unibo.it/knapsack.html) - Silvano Martello and Paolo Toth
 * [Lecture Notes of Linear Algebra](http://home.iitk.ac.in/~psraj/mth102/lecture_notes.html) - Dr. P. Shunmugaraj, IIT Kanpur (PDF)
 * [Linear Algebra](http://home.iitk.ac.in/~arlal/book/nptel/pdf/booklinear.html) - Dr. Arbind K Lal, IIT Kanpur (PDF)
 * [Linear Algebra](https://www.math.ucdavis.edu/~linear/linear-guest.pdf) (PDF)
-* [Linear Algebra: An Introduction to Mathematical Discourse](https://en.wikibooks.org/wiki/Linear_Algebra) - Jim Hefferon
+* [Linear Algebra by Jim Hefferon](http://joshua.smcvt.edu/linearalgebra) - Jim Hefferon
 * [Mathematical Logic - an Introduction](http://www.ii.uib.no/~michal/und/i227/book/book.pdf) (PDF)
 * [Mathematics, MTH101A](http://home.iitk.ac.in/~psraj/mth101/) - P. Shunmugaraj, IIT Kanpur
 * [Number Theory](https://github.com/holdenlee/number-theory) - Holden Lee MIT