fix typo in matrix_mult

Typo in expected functions:
right function name is row:

subjects/matrix_mult/README.md

@@ -21,7 +21,7 @@ impl Matrix<T> {
 	pub fn number_of_rows(&self) -> usize {
 	}
 
-	pub fn rows(&self, n: usize) -> Vec<T> {
+	pub fn row(&self, n: usize) -> Vec<T> {
 	}
 
 	pub fn col(&self, n: usize) -> Vec<T> {