try_unfold: Fix function docs wording

The wording of the documentation of the `try_unfold()` function was a
bit misleading.

The function signature clearly states that the closure that must be
provided to the function must return a `TryFuture` that yields an
`Option<(Item, T)>`.
The wording of the related documentation indicated that the closure may
return `None` or `Some(TryFuture)`, which is wrong.

This patch fixes that wording.

Signed-off-by: Matthias Beyer <[email protected]>

futures-util/src/stream/try_stream/try_unfold.rs

@@ -17,7 +17,7 @@ use pin_project_lite::pin_project;
 /// wait for the returned `TryFuture` to complete with `(a, b)`. It will then
 /// yield the value `a`, and use `b` as the next internal state.
 ///
-/// If the closure returns `None` instead of `Some(TryFuture)`, then the
+/// If the `TryFuture` returns `None` instead of `Some((Item, T))`, then the
 /// `try_unfold()` will stop producing items and return `Poll::Ready(None)` in
 /// future calls to `poll()`.
 ///