Fix doc build

[Naymoll]

doc_auto_cfg was merged into doc_cfg in rust-lang/rust#138907 and removed in Rust 1.92.0.

This PR fix the docs compilation failure

[INFO] [stderr] error[E0557]: feature has been removed
[INFO] [stderr]  --> src/lib.rs:1:29
[INFO] [stderr]   |
[INFO] [stderr] 1 | #![cfg_attr(docsrs, feature(doc_auto_cfg, doc_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`
[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 `tower-reqwest`