Fix doc test for Vec::retain(), now passes clippy::eval_order_dependence
Fix doc test for Vec::retain(), now passes clippy::eval_order_dependence

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