Replace `s` with `self` in docs for str methods taking self.
Replace `s` with `self` in docs for str methods taking self.

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