fix(docs): rename doc_auto_cfg attribute

See rust-lang/rust#138907

Author: jeertmans

src/lib.rs

@@ -2,7 +2,7 @@
 #![deny(missing_debug_implementations)]
 #![warn(clippy::must_use_candidate)]
 #![allow(clippy::doc_markdown, clippy::module_name_repetitions)]
-#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![cfg_attr(docsrs, feature(doc_cfg))]
 #![doc = include_str!("../README.md")]
 //!
 //! ## Note