Fix link capitalization in documentation of std::io::BufWriter.
Fix link capitalization in documentation of std::io::BufWriter.

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