Fix docs.rs build

Nightly feature doc_auto_cfg is merged into doc_cfg rust-lang/rust#138907

Author: NobodyXu

src/lib.rs

@@ -92,7 +92,7 @@
 #![deny(missing_debug_implementations)]
 #![allow(trivial_numeric_casts)]
 #![cfg_attr(test, deny(warnings))]
-#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![cfg_attr(docsrs, feature(doc_cfg))]
 
 #[cfg(not(feature = "any_impl",))]
 compile_error!("You need to choose a zlib backend");