typo fix and add extra contents

pages/Math4111/Math4111_L8.md

@@ -10,7 +10,7 @@ It should be empty. Proof any point cannot be in two balls at the same time. (By
 
 ### Metric space defs
 
-1. $p\in X,r>0$, $B_r(p)=\{q\in X:d(p,q)<0\}$, also called **neighborhood**.
+1. $p\in X,r>0$, $B_r(p)=\{q\in X:d(p,q)<r\}$, also called **neighborhood**.
 2. $p$ is a **limit point** of $E(p\in E')$ if $\forall r>0$, $(B_s(p)\cap E)\backslash \{p\}\neq \phi$
 3. If $p\in E$ and $p$ is not a limit point of $E$, then $p$ is called an **isolated point** of $E$.
 4. $E$ is **closed** if $E'\subset E$