Fix parameter names in std::env documentation.
Fix parameter names in std::env documentation.

The function parameters were renamed, but the documentation wasn't.

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