docs(pnet): use backticks for referencing types (#3252)

With the update to Rust 1.66, rustdoc is (rightfully) complaining about us writing bad HTML. `<u8>` is interpreted as an unclosed HTML tag. See https://github.com/libp2p/rust-libp2p/actions/runs/3708901592/jobs/6286942010#step:6:1380 for example.

transports/pnet/src/crypt_writer.rs

@@ -55,7 +55,7 @@ impl<W: AsyncWrite> CryptWriter<W> {
     }
 }
 
-/// Write the contents of a Vec<u8> into an AsyncWrite.
+/// Write the contents of a [`Vec<u8>`] into an [`AsyncWrite`].
 ///
 /// The handling 0 byte progress and the Interrupted error was taken from BufWriter in async_std.
 ///