Docs: replace broken link in api/README.md (#15423)
Docs: replace broken link in api/README.md (#15423)

In the api README a link is given for "API overfiew for users" that
leads to a 404.  It looks to be pointing to a v2_overview page that no
longer exists.  I replaced this link with the current overview at:
https://www.envoyproxy.io/docs/envoy/latest/configuration/overview/overview

Signed-off-by: Zach Mandeville <d7dacae2c968388960bf8970080a980ed5c5dcb7@ii.coop>
envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy