fix(doc): un-break docs.rs build

the `doc_auto_cfg` feature has been merged into `doc_cfg` [1], and this
is breaking building vm-memory documentation on docs.rs [2].

[1]: rust-lang/rust#138907
[2]: https://docs.rs/crate/vm-memory/0.17.0/builds/2564760

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

src/lib.rs

@@ -18,7 +18,7 @@
 #![warn(clippy::doc_markdown)]
 #![warn(missing_docs)]
 #![warn(missing_debug_implementations)]
-#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![cfg_attr(docsrs, feature(doc_cfg))]
 
 // We only support 64bit. Fail build when attempting to build other targets
 #[cfg(not(target_pointer_width = "64"))]