Don't require an explicit return type in the task macro

examples/tracing_example.rs

@@ -33,7 +33,7 @@
 //! - **ERROR**: Errors, panics, cycle detection
 //!
 
-use dagx::{task, DagRunner, Task};
+use dagx::{task, DagRunner};
 use futures::FutureExt;
 use tracing_subscriber::{fmt, EnvFilter};