Change from doc_auto_cfg to doc_cfg feature (#3428)

Fixes docs publishing by switching to the new feature merged in
rust-lang/rust#138907

Author: mkrasnitski

src/lib.rs

@@ -49,7 +49,7 @@
 //! [examples]: https://github.com/serenity-rs/serenity/tree/current/examples
 //! [gateway docs]: crate::gateway
 #![doc(html_root_url = "https://docs.rs/serenity/*")]
-#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![cfg_attr(docsrs, feature(doc_cfg))]
 #![warn(
     unused,
     rust_2018_idioms,