examples.md: Actually use `ctx` variable in Go example

"List and manage containers" Go example:
`ctx := context.Background()` was defined but never used.
Follow the structure of the other examples instead.

engine/api/sdk/examples.md

@@ -245,7 +245,7 @@ func main() {
 		panic(err)
 	}
 
-	containers, err := cli.ContainerList(context.Background(), types.ContainerListOptions{})
+	containers, err := cli.ContainerList(ctx, types.ContainerListOptions{})
 	if err != nil {
 		panic(err)
 	}