Fix doc build (#125)

Author: linyihai

src/lib.rs

@@ -37,7 +37,7 @@
 #![doc(
     html_logo_url = "https://raw.githubusercontent.com/smol-rs/smol/master/assets/images/logo_fullsize_transparent.png"
 )]
-#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg))]
 
 use std::fmt;
 use std::marker::PhantomData;