rust example build failed

[jim3333]

error[E0308]: mismatched types
   --> src/main.rs:537:13
    |
537 |             caller,
    |             ^^^^^^
    |             |
    |             expected enum `std::option::Option`, found struct `candid::Principal`
    |             help: try using a variant of the expected enum: `_::_serde::__private::Some(caller)`
    |
    = note: expected enum `std::option::Option<candid::Principal>`
             found struct `candid::Principal`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `token` due to previous error

ben@u2 ~/D/rust (main) [101]> rustc --version
rustc 1.57.0 (f1edd0429 2021-11-29)

[jim3333]

can use github actions to test automatically when pr.

[ferencdg]

can use github actions to test automatically when pr.

sure we are planning to add that, thanks for the PR