doc: remove out-date introduction video link. (etcd-io#11601)

It's easy to find etcd introduction video, and the introduction video
from the rfc doc is outdated, so removing this link.

Fixes 11591.

Documentation/rfc/v3api.md

@@ -2,7 +2,7 @@
 title: Overview
 ---
 
-The etcd v3 API is designed to give users a more efficient and cleaner abstraction compared to etcd v2. There are a number of semantic and protocol changes in this new API. For an overview [see Xiang Li's video](https://youtu.be/J5AioGtEPeQ?t=211).
+The etcd v3 API is designed to give users a more efficient and cleaner abstraction compared to etcd v2. There are a number of semantic and protocol changes in this new API.
 
 To prove out the design of the v3 API the team has also built [a number of example recipes](https://github.com/coreos/etcd/tree/master/contrib/recipes), there is a [video discussing these recipes too](https://www.youtube.com/watch?v=fj-2RY-3yVU&feature=youtu.be&t=590).