Rollup merge of rust-lang#117037 - csditchfield:fix_doc_writing_resul…

…t_example, r=notriddle

rustdoc book doc example error

closes rust-lang#117036

This is the minimal change required to make the second what-to-include.md example valid.
Another more modern solution could be considered:
```
/// Example
/// ```rust
/// let fortytwo = "42".parse::<u32>()?;
/// println!("{} + 10 = {}", fortytwo, fortytwo+10);
/// #     Ok::<(), <u32 as std::str::FromStr>::Err>(())
/// ```
```

src/doc/rustdoc/src/write-documentation/what-to-include.md

@@ -73,7 +73,7 @@ and your test suite, this example needs some additional code:
 ``````text
 /// Example
 /// ```rust
-/// # main() -> Result<(), std::num::ParseIntError> {
+/// # fn main() -> Result<(), std::num::ParseIntError> {
 /// let fortytwo = "42".parse::<u32>()?;
 /// println!("{} + 10 = {}", fortytwo, fortytwo+10);
 /// #     Ok(())