StringReader in `the-parser.md`

> [StringReader](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/lexer/struct.StringReader.html) from [rustc_ast](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/index.html) integrates rustc_lexer with rustc specific data structures.

StringReader doesn't come from `rustc_ast`.