Fix example in subsets.tex

Author: Weihnachtsbaum

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

@@ -26,7 +26,7 @@
 
 \begin{ex}
 Every set is a subset of itself, and $\emptyset$ is a subset of every
-set. The set of even numbers is a subset of the set of natural
+set. The set of natural even numbers is a subset of the set of natural
 numbers. Also, $\{ a, b \} \subseteq \{ a, b, c \}$. But $\{ a, b, e
 \}$ is not a subset of $\{ a, b, c \}$.
 \end{ex}