remove tokio mention

Signed-off-by: Yoshua Wuyts <yoshuawuyts@gmail.com>

src/stream/interval.rs

@@ -85,7 +85,7 @@ impl Stream for Interval {
 /// While technically for large duration it's impossible to represent any
 /// duration as nanoseconds, the largest duration we can represent is about
 /// 427_000 years. Large enough for any interval we would use or calculate in
-/// tokio.
+/// async-std.
 fn duration_to_nanos(dur: Duration) -> Option<u64> {
     dur.as_secs()
         .checked_mul(1_000_000_000)