timer: have example use delay_for instead of delay
It is a more common use case that is to simply cause a delay for an amount of time.
I think it is more appropriate to show off `delay_for` in the example rather than `delay` that is useful only for less common use cases.
//! Wait 100ms and print "Hello World!"
//!
//! ```
-//! use tokio::timer::delay;
+//! use tokio::timer::delay_for;
//!
//! use std::time::Duration;
//!
//!
//! #[tokio::main]
//! async fn main() {
-//! let when = tokio::clock::now() + Duration::from_millis(100);
-//! delay(when).await;
+//! delay_for(Duration::from_millis(100)).await;
//! println!("100 ms have elapsed");
//! }
//! ```
