update notations and fix typos

pages/Math4121/Math4121_L14.md

@@ -26,7 +26,7 @@ Proof:
 
 To prove Riemann's Integrability Criterion, we need to show that a bounded function $f$ is Riemann integrable if and only if for every $\sigma, \epsilon > 0$, there exists a partition $P$ of $[a, b]$ such that the sum of the lengths of the intervals where the oscillation exceeds $\sigma$ is less than $\epsilon$.
 
-EOP
+QED
 
 #### Proposition 2.4