Fixing ImageList Go code

Original: on line 18 we have `images, err := cli.ImageList(context.Background(), types.ImageListOptions{})` which gives us an error of:

```
# command-line-arguments
./main.go:12:2: ctx declared but not used
```

*Fix:* on line 18 we need to have `images, err := cli.ImageList(ctx, types.ImageListOptions{})`

engine/api/sdk/examples.md

@@ -471,7 +471,7 @@ func main() {
 		panic(err)
 	}
 
-	images, err := cli.ImageList(context.Background(), types.ImageListOptions{})
+	images, err := cli.ImageList(ctx, types.ImageListOptions{})
 	if err != nil {
 		panic(err)
 	}