fix: Missing 'main' function

Author: Laegel

docs/en/usage/guides/command.md

@@ -134,7 +134,7 @@ async fn my_custom_command<M: tauri::Params>(window: tauri::Window<M>) {
 
 Any or all of the above features can be combined:
 
-```rust
+```rust title=main.rs
 // Definition in main.rs
 
 #[derive(serde::Serialize)]
@@ -159,6 +159,13 @@ async fn my_custom_command<M: tauri::Params>(
     Err("No result".into())
   }
 }
+
+fn main() {
+  tauri::Builder::default()
+    .invoke_handler(tauri::generate_handler![my_custom_command])
+    .run(tauri::generate_context!())
+    .expect("error while running tauri application");
+}
 ```
 
 ```js