Rename `syntax` in librustc_ast/README.md
Rename `syntax` in librustc_ast/README.md
aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust