While-Let Unable to compile code example on page

[stephenlb]

Added fn main() as rustdoc didn't catch omitted main() function. There may be a simpler change by adding an annotations.

https://doc.rust-lang.org/rust-by-example/flow_control/while_let.html

   Compiling playground v0.0.1 (/playground)
error: expected item, found keyword `let`
 --> src/main.rs:2:1
  |
2 | let mut optional = Some(0);
  | ^^^ expected item
  |
  = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>

error: could not compile `playground` (bin "playground") due to 1 previous error

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @marioidival (or someone else) some time within the next two weeks.

[stephenlb]

Hi! Checking in to see if we can merge? Thank you! 😊

[stephenlb]

🎉