change edition to 2021 in exercices.rs

workaround for this issue #1022

Author: gjazeron

src/exercise.rs

@@ -121,7 +121,7 @@ impl Exercise {
                     r#"[package]
 name = "{}"
 version = "0.0.1"
-edition = "2018"
+edition = "2021"
 [[bin]]
 name = "{}"
 path = "{}.rs""#,