Fix rustdoc::broken_intra_doc_links warning

```
error: unresolved link to `Box`
  --> futures-util/src/future/always_ready.rs:43:83
   |
43 | /// This is particularly useful in avoiding a heap allocation when an API needs [`Box<dyn Future<Output = T>>`],
   |                                                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `Box` in scope
   |
   = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
   = note: `-D rustdoc::broken-intra-doc-links` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(rustdoc::broken_intra_doc_links)]`

error: unresolved link to `stream.take`
  --> futures-util/src/stream/repeat_with.rs:43:7
   |
43 | /// [`stream.take()`], in order to make them finite.
   |       ^^^^^^^^^^^^^ no item named `stream.take` in scope
   |
   = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

error: unresolved link to `stream.repeat`
  --> futures-util/src/stream/repeat_with.rs:47:11
   |
47 | /// the [`stream.repeat()`] function.
   |           ^^^^^^^^^^^^^^^ no item named `stream.repeat` in scope
   |
   = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

error: unresolved link to `futures-util::stream::select`
   --> futures-util/src/stream/select_with_strategy.rs:120:46
    |
120 | /// Note: this special case is provided by [`futures-util::stream::select`].
    |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `futures-util` in scope
```

Author: taiki-e

futures-util/src/future/always_ready.rs

@@ -40,7 +40,7 @@ impl<T, F: Fn() -> T> Future for AlwaysReady<T, F> {
 
 /// Creates a future that is always immediately ready with a value.
 ///
-/// This is particularly useful in avoiding a heap allocation when an API needs [`Box<dyn Future<Output = T>>`],
+/// This is particularly useful in avoiding a heap allocation when an API needs `Box<dyn Future<Output = T>>`,
 /// as [`AlwaysReady`] does not have to store a boolean for `is_finished`.
 ///
 /// # Examples

futures-util/src/stream/repeat_with.rs

@@ -40,11 +40,11 @@ impl<A, F: FnMut() -> A> FusedStream for RepeatWith<F> {
 /// The `repeat_with()` function calls the repeater over and over again.
 ///
 /// Infinite stream like `repeat_with()` are often used with adapters like
-/// [`stream.take()`], in order to make them finite.
+/// [`stream.take()`](crate::stream::StreamExt::take), in order to make them finite.
 ///
 /// If the element type of the stream you need implements [`Clone`], and
 /// it is OK to keep the source element in memory, you should instead use
-/// the [`stream.repeat()`] function.
+/// the [`stream::repeat()`](crate::stream::repeat) function.
 ///
 /// # Examples
 ///

futures-util/src/stream/select_with_strategy.rs

@@ -112,7 +112,7 @@ pin_project! {
 ///
 /// ### Round Robin
 /// This example shows how to select from both streams round robin.
-/// Note: this special case is provided by [`futures-util::stream::select`].
+/// Note: this special case is provided by [`stream::select`](crate::stream::select).
 ///
 /// ```rust
 /// # futures::executor::block_on(async {