fix typo in {fol}{seq}{prv}; closes #361

Author: rzach

content/first-order-logic/sequent-calculus/provability-consistency.tex

@@ -91,7 +91,7 @@
     \RightLabel{\LeftR{\Exchange}}
     \UnaryInf$!A, \lnot !A \fCenter$
     \RightLabel{\Cut}
-    \BinaryInf$\Gamma, \lnot !A \fCenter$
+    \BinaryInf$\Gamma_0, \lnot !A \fCenter$
   \end{prooftree}
   Since $\lnot !A \in \Gamma$ and $\Gamma_0 \subseteq \Gamma$, this
   shows that $\Gamma$ is inconsistent.