docs: fix docs.rs build

The current documentation build in docs.rs breaks with the following
error:

    [INFO] [stderr] error[E0557]: feature has been removed
    [INFO] [stderr]   --> src/lib.rs:11:29
    [INFO] [stderr]    |
    [INFO] [stderr] 11 | #![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`

So replace `doc_auto_cfg` with `doc_cfg`, which generates the expected
docs, showing the required features for each module.

Signed-off-by: Carlos López <carlos.lopezr4096@gmail.com>

Author: 00xc

CHANGELOG.md

@@ -1,5 +1,7 @@
 # Upcoming Release
 
+- [[#224](https://github.com/rust-vmm/linux-loader/pull/224)] Fixed docs.rs build.
+
 # [v0.13.2]
 
 ## Changed

src/lib.rs

@@ -8,7 +8,7 @@
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
 #![deny(missing_docs)]
-#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![cfg_attr(docsrs, feature(doc_cfg))]
 
 //! A Linux kernel image loading crate.
 //!