Fix a logging call.

Change-Id: I6c5937b942242819f63b8271c85a6aa41699e12d
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63410
Reviewed-by: Alejandro Sedeño <asedeno@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>