Add imports for *-mono-<= exercise (#1080)

Co-authored-by: Jefferson Carpenter <jefferson@aoeu2code.com>

Author: negatratoron

src/plfa/part1/Relations.lagda.md

@@ -15,8 +15,8 @@ the next step is to define relations, such as _less than or equal_.
 ```agda
 import Relation.Binary.PropositionalEquality as Eq
 open Eq using (_≡_; refl; cong)
-open import Data.Nat using (ℕ; zero; suc; _+_)
-open import Data.Nat.Properties using (+-comm; +-identityʳ)
+open import Data.Nat using (ℕ; zero; suc; _+_; _*_)
+open import Data.Nat.Properties using (+-comm; +-identityʳ; *-comm)
 ```