[DOC] Fixed Mesos version in doc from 0.18.1 to 0.21.0

#3934 upgraded Mesos version so we should also fix docs right?

This issue is really minor so I don't file in JIRA.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #3982 from sarutak/fix-mesos-version and squashes the following commits:

9a86ee3 [Kousuke Saruta] Fixed mesos version from 0.18.1 to 0.21.0