fix: remove unavailable video

- The video "Design Stock Exchange System" is no longer available on
  youtube. I believe it is private now.
- video link: https://www.youtube.com/watch?v=dUMWMZmMsVE
- section link: https://github.com/abdulrahim2002/awesome-system-design-resources#medium

README.md

@@ -109,7 +109,6 @@ This repository contains free resources to learn System Design concepts and prep
 - [Design Distributed Message Queue like Kafka](https://www.youtube.com/watch?v=iJLL-KPqBpM)
 - [Design Flight Booking System](https://www.youtube.com/watch?v=qsGcfVGvFSs)
 - [Design Online Code Editor](https://www.youtube.com/watch?v=07jkn4jUtso)
-- [Design Stock Exchange System](https://www.youtube.com/watch?v=dUMWMZmMsVE)
 - [Design an Analytics Platform (Metrics & Logging)](https://www.youtube.com/watch?v=kIcq1_pBQSY)
 - [Design Payment System](https://www.youtube.com/watch?v=olfaBgJrUBI)
 - [Design a Digital Wallet](https://www.youtube.com/watch?v=4ijjIUeq6hE)