[usecase-webapi][XWALK-2406]Update test steps description for Content

- Fail Reason: When ran getDirectories method, neither callback was invoked.

Impacted TCs num(approved): New 0, Update 1, Delete 0
Unit test Platform: [Tizen IVI]
Unit test result summary: Pass 0, Fail 1, Blocked 0
Merge pull request #792 from JianfengXu/xujf/2406

[usecase-webapi][XWALK-2406]Update test steps description for Content