Tweak `write_fmt` doc.
Tweak `write_fmt` doc.

Previous version wrongly used `but` while the two parts of the sentence are not contradicting but completing with each other.
graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust