fixed typo

inputs/lecture_03.tex

@@ -167,7 +167,7 @@
         Since $X$ is second countable and Hausdorff,
         we get the upper bound.%
     }{Lower bound:  $2^{\N} \hookrightarrow X$,
-        upper bound: \nth{2} countable and Hausdorff.
+        upper bound: \nth{2} countable and Hausdorff.}
 \end{proof}
 
 \begin{theorem}