Fix an instance of not doc(cfg(.*))

This unbreaks the CI in another crates that use
`RUSTDOCFLAGS=--cfg=docsrs` to verify correctness of their own things. I
wonder why docs.rs itself does not fail on this though.

(cherry picked from commit 25e8f911357c740034f10a170dfa4ea1b28234ce)
diff --git a/src/macros.rs b/src/macros.rs
index db93dfd..f97f909 100644
--- a/src/macros.rs
+++ b/src/macros.rs
@@ -52,7 +52,7 @@
     ($($item:item)*) => {
         $(
             #[cfg(any(feature = "net", all(unix, feature = "os-ext")))]
-            #[cfg_attr(docsrs, doc(any(feature = "net", all(unix, feature = "os-ext"))))]
+            #[cfg_attr(docsrs, doc(cfg(any(feature = "net", all(unix, feature = "os-ext")))))]
             $item
         )*
     }