Use the right kind of doc comment in chains.rs
Use the right kind of doc comment in chains.rs

cc #2185

graydon/rust,SingingTree/rustfmt,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,rust-lang/rustfmt,SingingTree/rustfmt,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,rust-lang/rustfmt