rename std::vec_ng -> std::vec

Closes #12771

src/librustdoc/html/toc.rs

@@ -11,7 +11,7 @@
 //! Table-of-contents creation.
 
 use std::fmt;
-use std::vec_ng::Vec;
+use std::vec::Vec;
 
 /// A (recursive) table of contents
 #[deriving(Eq)]