trpl: Fix example logic in error handling chapter

src/doc/book/error-handling.md

@@ -681,6 +681,7 @@ fn double_arg(mut argv: env::Args) -> Result<i32, String> {
     argv.nth(1)
         .ok_or("Please give at least one argument".to_owned())
         .and_then(|arg| arg.parse::<i32>().map_err(|err| err.to_string()))
+        .map(|n| 2 * n)
 }
 
 fn main() {