fix error Δ2 -> Δ1

Author: yiannis-had

content/normal-modal-logic/completeness/frame-completeness.tex

@@ -97,7 +97,7 @@
   $R^\Sigma \Delta_2\Delta_3$. The first hypothesis gives
   $\Box^{-1}\Delta_1 \subseteq \Delta_2$\iftag{prvBox}{}{ by
     \olref[mod]{lem:box-iff-diamond}}, and the second hypothesis is
-  equivalent to $\Diamond\Delta_3 \subseteq \Delta_2$\iftag{prvBox}{,
+  equivalent to $\Diamond\Delta_3 \subseteq \Delta_1$\iftag{prvBox}{,
     by \olref[mod]{lem:box-iff-diamond}}{}.  To show $R^\Sigma
   \Delta_2\Delta_3$\iftag{prvBox}{, by
     \olref[mod]{lem:box-iff-diamond}, it suffices to}{ we have to}