Rollup merge of #41886 - RalfJung:unique-doc-typo, r=steveklabnik
Rollup merge of #41886 - RalfJung:unique-doc-typo, r=steveklabnik

fix typo in Unique::empty doc

Subject says it all

GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust