Use the current_thread scheduler in doctest

src/writer/async_tokio.rs

@@ -69,7 +69,7 @@ impl<W: AsyncWrite + Unpin> Writer<W> {
     ///
     /// ```rust
     /// # use quick_xml::Result;
-    /// # #[tokio::main] async fn main() -> Result<()> {
+    /// # #[tokio::main(flavor = "current_thread")] async fn main() -> Result<()> {
     /// use quick_xml::events::{BytesStart, BytesText, Event};
     /// use quick_xml::writer::Writer;
     /// use quick_xml::Error;