slog: fix HandlerOptions.ReplaceAttr doc

THe docs says that level is omitted if zero, but it is never omitted.

Change-Id: Ie64423e3450dbda59fed2f21e2ae7fdcf7d32d15
Reviewed-on: https://go-review.googlesource.com/c/exp/+/464035
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
diff --git a/slog/handler.go b/slog/handler.go
index 7c2a21b..cd0f911 100644
--- a/slog/handler.go
+++ b/slog/handler.go
@@ -129,8 +129,8 @@
 	// If ReplaceAttr returns an Attr with Key == "", the attribute is discarded.
 	//
 	// The built-in attributes with keys "time", "level", "source", and "msg"
-	// are passed to this function, except that time and level are omitted
-	// if zero, and source is omitted if AddSourceLine is false.
+	// are passed to this function, except that time is omitted
+	// if zero, and source is omitted if AddSource is false.
 	//
 	// The first argument is a list of currently open groups that contain the
 	// Attr. It must not be retained or modified. ReplaceAttr is never called