Add missing dyn

Author: alcuadrado

src/error/multiple_error_types/reenter_question_mark.md

@@ -26,8 +26,8 @@ Here, we rewrite the previous example using `?`. As a result, the
 use std::error;
 use std::fmt;
 
-// Change the alias to `Box<error::Error>`.
-type Result<T> = std::result::Result<T, Box<error::Error>>;
+// Change the alias to `Box<dyn error::Error>`.
+type Result<T> = std::result::Result<T, Box<dyn error::Error>>;
 
 #[derive(Debug)]
 struct EmptyVec;