fix(data/matrix/basic): remove an accidental requirement for a matrix…

… to be square (#8372)

src/data/matrix/basic.lean

@@ -353,7 +353,7 @@ instance [has_mul α] [add_comm_monoid α] : has_mul (matrix n n α) := ⟨matri
 @[simp] theorem mul_eq_mul [has_mul α] [add_comm_monoid α] (M N : matrix n n α) :
   M * N = M ⬝ N := rfl
 
-theorem mul_apply' [has_mul α] [add_comm_monoid α] {M N : matrix n n α} {i k} :
+theorem mul_apply' [has_mul α] [add_comm_monoid α] {M : matrix l m α} {N : matrix m n α} {i k} :
   (M ⬝ N) i k = dot_product (λ j, M i j) (λ j, N j k) := rfl
 
 @[simp] theorem diagonal_neg [decidable_eq n] [add_group α] (d : n → α) :