Use --master-data, not --dump-slave, in bin/storage dump

Summary: These flags do slightly different things, I actually want --master-data here. My test databases are setup half-weird and work with either statement, which is why I missed this.

Test Plan: Ran a dump against master, got the right CHANGE MASTER statement with no warnings.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15716