[usecase-webapi][XWALK-2252] Correct the description of 5th subcase about Flexible

Impacted tests(approved): new 0, update 1, delete 0
Unit test platform: [Tizen IVI][Android]
Unit test result summary: pass 1, fail 0, block 0
Merge pull request #649 from zhuyongyong/yongzhu/usecase-webapi/flexibleBox-bug

[usecase-webapi][XWALK-2252] Correct the description about Flexible