Replace docs.rs build doc_auto_cfg feature with doc_cfg

Unfortunately, `doc_auto_cfg` was removed, breaking doc builds for
v0.7.0-rc.0. Here we replace it with the `doc_cfg` attribute.

Author: tnull

src/lib.rs

@@ -75,7 +75,7 @@
 #![deny(rustdoc::private_intra_doc_links)]
 #![allow(bare_trait_objects)]
 #![allow(ellipsis_inclusive_range_patterns)]
-#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![cfg_attr(docsrs, feature(doc_cfg))]
 
 mod balance;
 mod builder;