fix: docs.rs build failure (#591) (#600)

See https://docs.rs/crate/object_store/latest/builds/2752265 :

```text
[INFO] [stderr]  Documenting object_store v0.13.0 (/opt/rustwide/workdir)
[INFO] [stderr] error[E0557]: feature has been removed
[INFO] [stderr]   --> src/lib.rs:18:29
[INFO] [stderr]    |
[INFO] [stderr] 18 | #![cfg_attr(docsrs, feature(doc_auto_cfg))]
[INFO] [stderr]    |                             ^^^^^^^^^^^^ feature has been removed
[INFO] [stderr]    |
[INFO] [stderr]    = note: removed in 1.92.0; see <rust-lang/rust#138907> for more information
[INFO] [stderr]    = note: merged into `doc_cfg`
[INFO] [stderr]
[INFO] [stderr] error: Compilation failed, aborting rustdoc
[INFO] [stderr]
[INFO] [stderr] For more information about this error, try `rustc --explain E0557`.
[INFO] [stderr] error: could not document `object_store`
```

Co-authored-by: Marco Neumann <marco@crepererum.net>

Author: alamb

src/lib.rs

@@ -15,7 +15,7 @@
 // specific language governing permissions and limitations
 // under the License.
 
-#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![cfg_attr(docsrs, feature(doc_cfg))]
 #![deny(rustdoc::broken_intra_doc_links, rustdoc::bare_urls, rust_2018_idioms)]
 #![warn(
     missing_copy_implementations,