Fix rustdoc::broken_intra_doc_links warning

```
warning: unresolved link to `String`
  --> src/lib.rs:47:36
   |
47 | //! Generate a random [`Vec`] or [`String`]:
   |                                    ^^^^^^ no item named `String` in scope
   |
   = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
   = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default
```

Author: taiki-e

src/lib.rs

@@ -44,7 +44,7 @@
 //! fastrand::shuffle(&mut v);
 //! ```
 //!
-//! Generate a random [`Vec`] or [`String`]:
+//! Generate a random [`Vec`] or [`String`](alloc::string::String):
 //!
 //! ```
 //! use std::iter::repeat_with;