tracing: fix wrong log level in warn! macro (#1930)

## Motivation

I was scrolling through the code in `tracing/src/macros.rs` and noticed
a possibly wrong log level in the `warn!` macro:

https://github.com/tokio-rs/tracing/blob/4e65750b13721fee7a7ac05b053e1b9c3d21244f/tracing/src/macros.rs#L1698

## Solution

This branch changes the level to `Level::WARN` in the incorrect macro
 arm.

Fixes: #1929

Co-authored-by: Nicolaas Geldenhuys <nicolaas.geldenhuys@cartrack.com>

tracing/src/macros.rs

@@ -1701,7 +1701,7 @@ macro_rules! warn {
     (%$($k:ident).+ = $($field:tt)*) => (
         $crate::event!(
             target: module_path!(),
-            $crate::Level::TRACE,
+            $crate::Level::WARN,
             { %$($k).+ = $($field)*}
         )
     );