update notations and fix typos

pages/Math4121/Math4121_L17.md

@@ -38,7 +38,7 @@ If $b\in T$, then by definition of $T$, $b \notin \psi(b)$, but $\psi(b) = T$, w
 
 If $b \notin T$, then $b \in \psi(b)$, which is also a contradiction since $b\in T$. Therefore, $2^S$ cannot have the same cardinality as $S$.
 
-EOP
+QED
 
 ### Back to Hankel's Conjecture