slog: fix RFC3339 regexp when offset is positive

Change-Id: I2f037bf3a211a8e969330e47fc0a2d499ec92f80
Reviewed-on: https://go-review.googlesource.com/c/exp/+/446156
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
diff --git a/slog/logger_test.go b/slog/logger_test.go
index 4d171b4..ee6624d 100644
--- a/slog/logger_test.go
+++ b/slog/logger_test.go
@@ -17,7 +17,7 @@
 	"time"
 )
 
-const timeRE = `\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}(Z|-\d{2}:\d{2})`
+const timeRE = `\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}(Z|[+-]\d{2}:\d{2})`
 
 func TestLogTextHandler(t *testing.T) {
 	var buf bytes.Buffer