Switch from doc_auto_cfg to doc_cfg (#1228)

`doc_auto_cfg` was recently removed from rustc. See
RustCrypto/traits#2028.

`doc_cfg` is also now automatic.

Author: tarcieri

zeroize/src/lib.rs

@@ -1,5 +1,5 @@
 #![no_std]
-#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![cfg_attr(docsrs, feature(doc_cfg))]
 #![doc(
     html_logo_url = "https://raw.githubusercontent.com/RustCrypto/media/6ee8e381/logo.svg",
     html_favicon_url = "https://raw.githubusercontent.com/RustCrypto/media/6ee8e381/logo.svg"