CasperTFG small typo: "...used to by..."

[paulhauner]

As mentioned in the cbc-casper gitter, there seems to be a little typo in the CasperTFG paper on line 504:

"The score, remember, is used to by the GHOST fork choice rule to choose between children of some block."

Direct link:
https://github.com/ethereum/research/blob/master/papers/CasperTFG/CasperTFG.tex#L504

PS. The paper was great, love your work!