Add FlexBasisPercent to CheatSheet
Add FlexBasisPercent to CheatSheet

Summary: Edited via in page editor -- add docs into the cheatsheet for the flex basis percent, now that it has been added

Reviewed By: adityasharat

Differential Revision: D35235026

fbshipit-source-id: 6674cfdb267e0350feaf83ceab149331be9ebb67

facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho