Minor typo fix. bu -> by
@@ -69,7 +69,7 @@ impl Subscriptions {
/// Creates new subscription for given subscriber.
///
/// Second parameter is a function that converts Subscriber sink into a future.
- /// This future will be driven to completion bu underlying event loop
+ /// This future will be driven to completion by the underlying event loop
/// or will be cancelled in case #cancel is invoked.
pub fn add<T, E, G, R, F>(&self, subscriber: Subscriber<T, E>, into_future: G) where
G: FnOnce(Sink<T, E>) -> R,
