Fix blkio statistics name in documentation

In the latest blkio isolator documentation the blkio statistics are shown to exist in the json path `statistics.blkio`:
http://mesos.apache.org/documentation/latest/isolators/cgroups-blkio/

In fact, they exist at the path `statistics.blkio_statistics`:
https://github.com/apache/mesos/blob/master/include/mesos/v1/mesos.proto#L1692

I'm not sure which is correct, but we should make the docs consistent with the implementation.

/CC gilbert