404 readme link

[ratmice]

The link in the readme in the id rules section is a 404. there is this function of the same name
JsonRenderer::id_from_item_inner in the rustdoc docs, but it wasn't clear to me that it had anything to do with what the id rules section was discussing.

[zjp-CN]

Thanks for telling me the API changes.

Actually, it's a implementation detail to rely on the ID rule. And it'd be mangled in the future. Well, they made the ID an u32.

• cc Consider mangling ID's in rustdoc-json rust-lang/rust#91609
• cc rustdoc-json: change item ID's repr from a string to an int rust-lang/rust#130078