Update feature doc_auto_cfg to doc_cfg

[Oyami-Srk]

doc_auto_cfg has been removed in the latest nightly, see rust-lang/rust#138907. This feature was merged into doc_cfg.

This fixes the rustdoc build in the latest nightly toolchain with docsrs feature enabled.

Otherwise, it will break like:

$ RUSTDOCFLAGS="--cfg docsrs" RUSTFLAGS="--cfg docsrs" cargo +nightly doc --all-features
 Documenting socket2 v0.6.0 (/Users/shiroko/Projects/socket2)
error[E0557]: feature has been removed
  --> src/lib.rs:57:29
   |
57 | #![cfg_attr(docsrs, feature(doc_auto_cfg))]
   |                             ^^^^^^^^^^^^ feature has been removed
   |
   = note: removed in CURRENT_RUSTC_VERSION; see <https://github.com/rust-lang/rust/pull/138907> for more information
   = note: merged into `doc_cfg`

error: Compilation failed, aborting rustdoc

For more information about this error, try `rustc --explain E0557`.
error: could not document `socket2`

[Thomasdezeeuw]

Thanks @Oyami-Srk. For some reason I can't actually merge this right now, only "auto-merge" it. No need to make any further changes though, I'll sort this on our side.