add running result for ? operator using in main function

src/basic/result-error/result.md

@@ -340,6 +340,21 @@ fn main() {
 }
 ```
 
+运行后会报错:
+```shell
+$ cargo run
+   ...
+   the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
+ --> src/main.rs:4:48
+  |
+3 | fn main() {
+  | --------- this function should return `Result` or `Option` to accept `?`
+4 |     let greeting_file = File::open("hello.txt")?;
+  |                                                ^ cannot use the `?` operator in a function that returns `()`
+  |
+  = help: the trait `FromResidual<Result<Infallible, std::io::Error>>` is not implemented for `()`
+```
+
 因为 `?` 要求 `Result<T, E>` 形式的返回值，而 `main` 函数的返回是 `()`，因此无法满足，那是不是就无解了呢？
 
 实际上 Rust 还支持另外一种形式的 `main` 函数：