Added measurability in separable metric spaces.

src/topology/uniform/metric.tex

@@ -13,7 +13,7 @@ The axioms of uniform spaces strongly resembles working in a metric space. In fa
     \end{enumerate}
     If $d$ satisfies the above and
     \begin{enumerate}
-        \item[(M)] For any $x, y \in X$ with $x \ne y$, $d(x, y)$
+        \item[(M)] For any $x, y \in X$ with $x \ne y$, $d(x, y) > 0$.
     \end{enumerate}
     then $d$ is a \textbf{metric}.
 \end{definition}