Fix documentation on docs.rs (#301)

We were using the `doc_auto_cfg` feature, but that's been renamed to just
`doc_cfg`.

Author: madsmtm

CHANGELOG.md

@@ -1,5 +1,7 @@
 # Unreleased
 
+- Fix documentation building on `docs.rs`.
+
 # 0.4.7
 
 - Added support for Android using the `ndk` crate.

src/lib.rs

@@ -2,7 +2,7 @@
 #![allow(clippy::needless_doctest_main)]
 #![deny(unsafe_op_in_unsafe_fn)]
 #![warn(missing_docs)]
-#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![cfg_attr(docsrs, feature(doc_cfg))]
 
 extern crate core;