rustdoc Readme link fix

[evelynmitchell]

In the Readme for rustdoc:
https://github.com/rust-lang/rust/tree/master/src/doc
the link for further information points to a 404:
https://github.com/rust-lang/rust/tree/master/src/doc

The correct link is:
https://github.com/rust-lang/rust/blob/master/src/doc/rustdoc/src/what-is-rustdoc.md