docs(CasperTFG.tex): fix typo

This closes ethereum#82.

papers/CasperTFG/CasperTFG.tex

@@ -501,7 +501,7 @@ \section{Casper the Friendly Ghost with Validator Rotation}
 
 We use $\mathcal{W}$ to represent the set of all weights mappings $W:\mathcal{V} \to \mathbb{R}_+$
 
-We redefine a block's ``score'' to use the weights in the parent block. The score, remember, is used to by the GHOST fork choice rule to choose between children of some block. Those children (naturally) have the same parent, and therefore will have scores determined by the latest messages from the same validators.
+We redefine a block's ``score'' to use the weights in the parent block. The score, remember, is used by the GHOST fork choice rule to choose between children of some block. Those children (naturally) have the same parent, and therefore will have scores determined by the latest messages from the same validators.
 
 \begin{defn}[Score of a block, for Casper the Friendly Ghost with validator rotation]
 \begin{align}