Update readings.md

Removed Sheldon Axler's "Linear Algebra Done Right (FREE)" because it is no longer free after the end of July 2020. It was made free temporarily due to COVID-19.

extras/readings.md

@@ -36,7 +36,6 @@ Name | Author(s)
 [Applied Discrete Structures](http://faculty.uml.edu/klevasseur/ads2/) | Alan Doerr, Ken Levasseur
 [Grinstead and Snell’s Introduction to Probability](https://math.dartmouth.edu/~prob/prob/prob.pdf) |Charles M. Grinstead, J. Laurie Snell
 [Introduction to Linear Algebra](https://www.amazon.com/Introduction-Linear-Algebra-Gilbert-Strang/dp/0980232775/) | Gilbert Strang
-[Linear Algebra Done Right (FREE)](https://link.springer.com/book/10.1007/978-3-319-11080-6) | Sheldon Axler
 
 ## Systems