chore: fix docs.rs builds

doc_auto_cfg has been merged into doc_cfg in rust-lang/rust#138907

Author: Dr-Emann

croaring/src/lib.rs

@@ -1,5 +1,5 @@
 #![no_std]
-#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![cfg_attr(docsrs, feature(doc_cfg))]
 #![deny(missing_docs)]
 //! Rust wrapper for `CRoaring` (a C/C++ implementation at <https://github.com/RoaringBitmap/CRoaring>)
 //!