Fixed regex incident.

src/topology/metric/metric.tex

@@ -9,7 +9,7 @@
         \item[(M)] For any $x, y \in X$ with $x \ne y$, $d(x, y) > 0$.
         \item[(PM2)] For any $x, y \in X$, $d(x, y) = d(y, x)$.
         \item[(PM3)] For any $x, y, z \in X$, $d(x, z) \le d(x, y) + d(y, z)$.
-    \end\{enumerate\}
+    \end{enumerate}
 
     The pair $(X, d)$ is a \textbf{metric space}, which comes with the metric uniformity induced by $d$, and the corresponding topology.
 \end{definition}