update Fx WOFF2 flag

https://developer.mozilla.org/en-US/docs/Web/CSS/@font-face#Browser_compatibility

NOTE: MDN has WOFF2 starting Chrome 38 & Opera 24, while your data has 36 & 23.  I can't test ATM, so I'll leave.
Merge pull request #812 from tomByrer/patch-1

update Fx WOFF2 flag