Docs: fix broken links to React Conf videos

the playlist links to the React conf 2019 & 2018 videos seem to be broken

<br>

**screenshots:**
![react conf 2019 missing video](https://user-images.githubusercontent.com/50571688/114677380-eca2da80-9d12-11eb-8b37-83c807c7e50b.jpg)

![react conf 2018 missing video](https://user-images.githubusercontent.com/50571688/114677492-09d7a900-9d13-11eb-9366-a782548dd86c.jpg)

