Disable threads test on wasm

Test fails here:
https://github.com/tokio-rs/bytes/actions/runs/16857088623/job/47751892067?pr=791

Author: stepancheg

src/bytes_mut.rs

@@ -226,6 +226,9 @@ impl BytesMut {
     /// # Examples
     ///
     /// ```
+    /// # // Threads do not work on wasm.
+    /// # #![cfg(not(target_family = "wasm"))]
+    ///
     /// use bytes::{BytesMut, BufMut};
     /// use std::thread;
     ///