Update src/liballoc/vec.rs
Update src/liballoc/vec.rs

Following suggestion from @jonas-schievink

Co-Authored-By: Jonas Schievink <1c14e31848555af9d3ec1a1211eb802a2c778d15@gmail.com>
aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust