Update rustdoc.md : rename pkgid to crate_id

Author: eliovir

doc/rustdoc.md

@@ -11,9 +11,7 @@ Documenting Rust APIs is quite simple. To document a given item, we have "doc
 comments":
 
 ~~~
-// the "link" crate attribute is currently required for rustdoc, but normally
-// isn't needed.
-#[pkgid = "universe"];
+#[crate_id = "universe"];
 #[crate_type="lib"];
 
 //! Tools for dealing with universes (this is a doc comment, and is shown on