Merge pull request #28921 from allencloud/remove-incorrect-url

update an incorrect url in docs

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/)