Fix typo in bisimulation definition

coalgebra.tex

@@ -45,7 +45,7 @@
   For a \de{endofunctor} \(F : \map{\Set}{\Set}\) and two \fca{}
   \((C, c)\), \((D, d)\), are \emph{behaviourally equivalent} for
   \(C \ni x \sim y \in D\) if
-  \[ \exists h, k \st (C, c) \xrightarrow{h} (E, d) \xleftarrow{k} (D,
+  \[ \exists h, k \st (C, c) \xrightarrow{h} (E, e) \xleftarrow{k} (D,
     d), \] for some \((E, e)\).
 \end{definition}