slog: make Info case in Level.String match the others

Since InfoLevel == 0, the code was correct.
But adding "- InfoLevel" makes it more obviously so.

Change-Id: I6b222fbae2bcc04d5e54a7319796c5e9347cf55d
Reviewed-on: https://go-review.googlesource.com/c/exp/+/447964
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/level.go b/slog/level.go
index e75691e..c613cf9 100644
--- a/slog/level.go
+++ b/slog/level.go
@@ -67,7 +67,7 @@
 	case l < InfoLevel:
 		return str("DEBUG", l-DebugLevel)
 	case l < WarnLevel:
-		return str("INFO", l)
+		return str("INFO", l-InfoLevel)
 	case l < ErrorLevel:
 		return str("WARN", l-WarnLevel)
 	default: