Fix docs.rs build

[survived]

I noticed that docs build fails on docs.rs after my PR #28 with this error message:

[INFO] [stderr] error[E0557]: feature has been removed
[INFO] [stderr]   --> src/lib.rs:10:29
[INFO] [stderr]    |
[INFO] [stderr] 10 | #![cfg_attr(docsrs, feature(doc_auto_cfg))]
[INFO] [stderr]    |                             ^^^^^^^^^^^^ feature has been removed
[INFO] [stderr]    |
[INFO] [stderr]    = note: removed in 1.92.0; see <https://github.com/rust-lang/rust/pull/138907> for more information
[INFO] [stderr]    = note: merged into `doc_cfg`

Replacing this feature with doc_cfg fixes an error. I checked that docs are building on latest nightly.