message: tracing: fix wrong log level in `warn!` macro
  ## Motivation
  I was scrolling through the code in `tracing/src/macros.rs` and noticed
  a possibly wrong log level in the `warn!` macro:
  ## Solution
  This branch changes the level to `Level::WARN` in the incorrect macro
  arm.
  Fixes:
diff: @@ -1695,7 +1695,7 @@ macro_rules! warn {
  (%$($k:ident).+ = $($field:tt)*) => (
  $crate::event!(
  target: module_path!(),
  - $crate::Level::TRACE,
  + $crate::Level::WARN,
  { %$($k).+ = $($field)*}
  )
  );
  