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

As mentioned in the [cbc-casper gitter](https://gitter.im/cbc-casper/Lobby), there seems to be a little typo in the [CasperTFG paper](https://github.com/ethereum/research/tree/master/papers/CasperTFG) 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!

Open papers/CasperTFG/CasperTFG.tex and inspect line 504, where the reported wording appears. Correct the typo in the sentence about the score and the GHOST fork choice rule; done means the sentence reads correctly in the paper source.

Written by the indexing model from the issue text.