Fixed typographical error, changed accross to across in README.
Fixed typographical error, changed accross to across in README.

tttpeng/FLEX,fbernardo/FLEX,BrianBatchelder/FLEX,Xiaobin0860/FLEX,tttpeng/FLEX-DatabaseBrowser,NSExceptional/FLEX,DaidoujiChen/FLEX,Automatic/FLEX,Flipboard/FLEX,programming086/FLEX,pjebs/FLEX,NSExceptional/FLEX,Flipboard/FLEX