[usecase-webapi] Fix bugs of Workers.

- Modify the button name 'Start' to 'Calculation'.

Impacted tests(approved): New 0, Update 1, Delete 0
Unit test platform: [Tizen IVI]
Unit test result summary: Pass 1, Fail 0, Block 0
I am not sure why this is a bug? Can you point me where is the bug report? Or send the email which says this is bug.