update and fix typo

pages/Math4121/Math4121_L18.md

@@ -1,4 +1,4 @@
-# Math4121 Lecture 18
+# Math 4121 Lecture 18
 
 ## Continue
 
@@ -10,7 +10,7 @@ By modifying this example, we can find similar with any outer content between 0
 
 #### Definition: Perfect Set
 
-$S\subsetes[0,1]$ is perfect if $S=S'$.
+$S\subseteq[0,1]$ is perfect if $S=S'$.
 
 Example:
 
@@ -28,7 +28,7 @@ Let $C_0=[0,1]$, $C_1=[0,\frac{1}{3}]\cup[\frac{2}{3}]$ ...
 Continuing this process indefinitely, we define the Cantor set as
 
 $$
-C=\Bigcap_{n=0}^{\infty}C_n
+C=\bigcap_{n=0}^{\infty}C_n
 $$
 
 1. $C_n\subseteq C_{n-1}$