fix: update api.go link in README.md (#709)

`internal/core/api.go` was moved to `pkg/api/api.go`
(1a7a413), so updating the link.

Fixes #675.

Signed-off-by: Paul Boyd <[email protected]>

README.md

@@ -70,7 +70,7 @@ For a high-level documentation of the Model Registry _logical model_, please che
 ## Model Registry Core
 
 The model registry core is the layer which implements the core/business logic by interacting with the underlying ml-metadata server.
-It provides a model registry domain-specific [api](internal/core/api.go) that is in charge to proxy all, appropriately transformed, requests to ml-metadata using gRPC calls.
+It provides a model registry domain-specific [api](pkg/api/api.go) that is in charge to proxy all, appropriately transformed, requests to ml-metadata using gRPC calls.
 
 ### Model registry library