docs: fix function comment

The comment on the `visit_spans` fn is wrong. Change it to the correct
order.

Fixes: tokio-rs#2940

tracing-subscriber/src/fmt/fmt_layer.rs

@@ -1041,9 +1041,9 @@ where
 {
     /// Visits every span in the current context with a closure.
     ///
-    /// The provided closure will be called first with the current span,
-    /// and then with that span's parent, and then that span's parent,
-    /// and so on until a root span is reached.
+    /// The provided closure will be called first with the root span,
+    /// and then traverse through all the children until it reaches 
+    /// the current one.
     pub fn visit_spans<E, F>(&self, mut f: F) -> Result<(), E>
     where
         F: FnMut(&SpanRef<'_, S>) -> Result<(), E>,