fix issue

officialaccount/material/material.go

@@ -4,11 +4,12 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"github.com/silenceper/wechat/v2/officialaccount/context"
-	"github.com/silenceper/wechat/v2/util"
 	"io"
 	"os"
 	"path"
+
+	"github.com/silenceper/wechat/v2/officialaccount/context"
+	"github.com/silenceper/wechat/v2/util"
 )
 
 const (