Fix error when building psk-exchange example

[metametapod]

Fixes the build error:

$ cargo build --example psk-exchange
error[E0433]: failed to resolve: could not find `main` in `tokio`
 --> talpid-tunnel-config-client/examples/psk-exchange.rs:7:10
  |
7 | #[tokio::main]
  |          ^^^^ could not find `main` in `tokio`

error[E0752]: `main` function is not allowed to be `async`
 --> talpid-tunnel-config-client/examples/psk-exchange.rs:8:1
  |
8 | async fn main() {
  | ^^^^^^^^^^^^^^^ `main` function is not allowed to be `async`

Some errors have detailed explanations: E0433, E0752.
For more information about an error, try `rustc --explain E0433`.
error: could not compile `talpid-tunnel-config-client` (example "psk-exchange") due to 2 previous errors

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This change is 

[faern]

Thanks for the fix! Will merge as soon as the CI passes.

[faern]

Thanks for the contribution.