Outdated link to trait `ResolverAstLowering` in Name Resolution chapter

[MihailMihov]

https://rustc-dev-guide.rust-lang.org/name-resolution.html#basics mentions a trait ResolverAstLowering and links to https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast_lowering/trait.ResolverAstLowering.html, which no longer exists since rust-lang/rust@603746a. The documentation for the struct ResolverAstLowering is located at https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.ResolverAstLowering.html, but I'm not sure how to reword the paragraph, so I'm just opening an issue about it.