typo; closes #420

Author: rzach

content/sets-functions-relations/relations/trees.tex

@@ -50,7 +50,7 @@
 A \emph{tree} is a pair $T = \tuple{A, \le}$ such that $A$ is a set
 and $\le$ is a partial order on~$A$ with a unique minimal element
 $r \in A$ (called the \emph{root}) such that for all $x \in A$,
-the set $\Setabs{y}{y \le z}$ is well-ordered by~$\le$.
+the set $\Setabs{y}{y \le x}$ is well-ordered by~$\le$.
 \end{defn}
 
 \begin{defn}[Successors]