subscriber: fix clippy::map_flatten lint (#1964)

Clippy now warns about code that calls `.map(...).flatten()` on an
iterator and suggests `.flat_map(...)` instead:
https://rust-lang.github.io/rust-clippy/master/index.html#map_flatten

This branch fixes one place where we do that. CI should be happy again.

tracing-subscriber/src/fmt/format/mod.rs

@@ -1129,8 +1129,7 @@ where
         for span in ctx
             .event_scope()
             .into_iter()
-            .map(crate::registry::Scope::from_root)
-            .flatten()
+            .flat_map(crate::registry::Scope::from_root)
         {
             let exts = span.extensions();
             if let Some(fields) = exts.get::<FormattedFields<N>>() {