update and fix typo

pages/Math4121/Math4121_L19.md

@@ -34,11 +34,7 @@ If $C$ is cover of $T$, then $S\subseteq T\subseteq C$, so $C$ is a cover of $S$
 
 QED
 
-#### Theorem Osgorod's Lemma
+#### Theorem Osgood's Lemma
 
-If $S$ is closed and bounded, then
-
-$$
-\lim_{k\to \infty} c_e(S_k)=c_e(S)
-$$
+Let $S$ be a closed, bounded set in $\mathbb{R}$, and $S_1\subseteq S_2\subseteq \ldots$, and $S=\bigcup_{n=1}^{\infty} S_n$. Then $\lim_{k\to\infty} c_e(S_k)=c_e(S)$.