fix(macos): fix `SingleInstanceCallback` type error (#4)

src/platform_impl/macos.rs

@@ -5,6 +5,6 @@ use tauri::{
     plugin::{self, TauriPlugin},
     Runtime,
 };
-pub fn init<R: Runtime>(f: Box<SingleInstanceCallback>) -> TauriPlugin<R> {
+pub fn init<R: Runtime>(f: Box<SingleInstanceCallback<R>>) -> TauriPlugin<R> {
     plugin::Builder::new("single-instance").build()
 }