doc: fix typo in OsStrExt and OsStringExt
doc: fix typo in OsStrExt and OsStringExt

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