fix(matrix_mult): fix package name mismatch

subjects/matrix_mult/README.md

@@ -37,6 +37,8 @@ impl Mul for Matrix<T> {
 Here is a program to test your function.
 
 ```rust
+use matrix_mult::*;
+
 fn main() {
 	let matrix: Matrix<u32> = Matrix(vec![vec![3, 6], vec![8, 0]]);
 	println!("{:?}", matrix.col(0));