Use heading for `std::prelude` and not `io::prelude`

The heading style for `std::prelude` is to be consistent with the
headings for `std` and `core`: `# The Rust Standard Library` and
`# The Rust Core Library`, respectively.

library/std/src/io/prelude.rs

@@ -1,4 +1,4 @@
-//! # The I/O Prelude
+//! The I/O Prelude.
 //!
 //! The purpose of this module is to alleviate imports of many common I/O traits
 //! by adding a glob import to the top of I/O heavy modules:

library/std/src/prelude/mod.rs

@@ -1,4 +1,4 @@
-//! The Rust Prelude.
+//! # The Rust Prelude
 //!
 //! Rust comes with a variety of things in its standard library. However, if
 //! you had to manually import every single thing that you used, it would be