Move /x/net/context to context in docker client README

Signed-off-by: Josh Chorlton <jchorlton@gmail.com>

client/README.md

@@ -11,10 +11,10 @@ package main
 
 import (
 	"fmt"
+	"context"
 
 	"github.com/docker/docker/client"
 	"github.com/docker/docker/api/types"
-	"golang.org/x/net/context"
 )
 
 func main() {