slog: fix example in Level.String doc

Change-Id: Id8faad8460c87dc736ad32c3bcc06ef36c26c33a
Reviewed-on: https://go-review.googlesource.com/c/exp/+/461095
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
diff --git a/slog/level.go b/slog/level.go
index f6588de..871c1ea 100644
--- a/slog/level.go
+++ b/slog/level.go
@@ -54,7 +54,7 @@
 // Examples:
 //
 //	LevelWarn.String() => "WARN"
-//	(LevelWarn-2).String() => "WARN-2"
+//	(LevelInfo+2).String() => "INFO+2"
 func (l Level) String() string {
 	str := func(base string, val Level) string {
 		if val == 0 {