Fix a typo (#258)

tex/measure/swapsum.tex

@@ -185,7 +185,7 @@ \section{Everything else}
 		&\le \int_\Omega f \; d\mu
 	\end{align*}
 	where the first $\le$ is by Fatou lemma,
-	and the second by the fact that
+	and the third by the fact that
 	$\int_\Omega f_n \le \int_\Omega f$ for every $n$.
 	This implies all the inequalities are equalities and we are done.
 \end{proof}