Corrects a typo in rustdoc documentation.
Corrects a typo in rustdoc documentation.

Fixes rust-lang#70856

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