docs(example): fix typo in upgrade example (hyperium#3479)

Signed-off-by: Sven Pfennig <s.pfennig@reply.de>

examples/upgrades.rs

@@ -37,7 +37,7 @@ async fn server_upgraded_io(upgraded: Upgraded) -> Result<()> {
 
     // and now write back the server 'foobar' protocol's
     // response...
-    upgraded.write_all(b"barr=foo").await?;
+    upgraded.write_all(b"bar=foo").await?;
     println!("server[foobar] sent");
     Ok(())
 }