update incorrect url in docs

Signed-off-by: allencloud <allen.sun@daocloud.io>
(cherry picked from commit 5ae95d6245d30751951418650331def487bf935b)
Signed-off-by: Victor Vieux <vieux@docker.com>

docs/reference/index.md

@@ -18,4 +18,4 @@ keywords: "Engine"
 * [Dockerfile reference](builder.md)
 * [Docker run reference](run.md)
 * [Command line reference](commandline/index.md)
-* [API Reference](api/index.md)
+* [API Reference](https://docs.docker.com/engine/api/)