fix(LinearAlgebra/Matrix/Notation): fix defeq abuse in lemma (#42089)

Co-authored-by: alreadydone <3064145+alreadydone@users.noreply.github.com>

Author: mathlib-splicebot[bot]

Mathlib/LinearAlgebra/Matrix/Notation.lean

@@ -401,7 +401,7 @@ section Submatrix
 
 @[simp]
 theorem submatrix_empty (A : Matrix m' n' α) (row : Fin 0 → m') (col : o' → n') :
-    submatrix A row col = ![] :=
+    submatrix A row col = of ![] :=
   empty_eq _
 
 set_option backward.isDefEq.respectTransparency false in