style(Data/Matrix/Block): define Matrix.fromBlocks more esthetically (

#19324)

Mathlib/Data/Matrix/Block.lean

@@ -40,7 +40,7 @@ dimensions. -/
 @[pp_nodot]
 def fromBlocks (A : Matrix n l α) (B : Matrix n m α) (C : Matrix o l α) (D : Matrix o m α) :
     Matrix (n ⊕ o) (l ⊕ m) α :=
-  of <| Sum.elim (fun i => Sum.elim (A i) (B i)) fun i => Sum.elim (C i) (D i)
+  of <| Sum.elim (fun i => Sum.elim (A i) (B i)) (fun j => Sum.elim (C j) (D j))
 
 @[simp]
 theorem fromBlocks_apply₁₁ (A : Matrix n l α) (B : Matrix n m α) (C : Matrix o l α)