Need to update pageToken to nextPageToken

[sofisl]

see https://buganizer.corp.google.com/issues/391720095#comment17

We need to update this type https://github.com/googleapis/nodejs-logging/blob/9d1d480406c4d1526c8a7fafd9b18379c0c7fcea/src/log.ts#L47 to actually match the correct token returned nextPageToken

[cindy-peng]

Thanks for reporting! Will work on the fix!