docs: fix runtime creation doc in tokio::runtime
With the rt-threaded feature flag we create a threaded scheduler by
default. The documentation had a copy-and-paste error from the section
about the basic scheduler.
//! ```
//!
//! If the `rt-threaded` feature flag is enabled, [`Runtime::new`] will return a
-//! basic scheduler runtime by default.
+//! threaded scheduler runtime by default.
//!
//! Most applications should use the threaded scheduler, except in some niche
//! use-cases, such as when running only a single thread is required.
