fix what-to-include doc example

Fixes the second example in the Examples section of what-to-include.md  by marking main as a function.

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(())