Fix std::future::from_generator documentation
Fix std::future::from_generator documentation

This function takes a generator and wraps it in a future, not
vice-versa.

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