Update index.md (#9078)

"io" needs to be imported, in order to avoid " undefined: io" error. 
(go version: go1.12.5 linux/amd64)

P.S: import statements are reordered/formatted (gofmt).

Cheers

develop/sdk/index.md

@@ -180,13 +180,14 @@ Docker API directly, or using the Python or Go SDK.
 package main
 
 import (
-    "context"
-    "os"
+	"context"
+	"io"
+	"os"
 
-    "github.com/docker/docker/client"
-    "github.com/docker/docker/api/types"
-    "github.com/docker/docker/api/types/container"
-    "github.com/docker/docker/pkg/stdcopy"
+	"github.com/docker/docker/api/types"
+	"github.com/docker/docker/api/types/container"
+	"github.com/docker/docker/client"
+	"github.com/docker/docker/pkg/stdcopy"
 )
 
 func main() {