Fix abandoned usage: use 'rand.Read' instead of 'rand.Reader' to generate a random string

zh/06.2.md

@@ -111,7 +111,7 @@ Session ID是用来识别访问Web应用的每一个用户，因此必须保证
 
 func (manager *Manager) sessionId() string {
 	b := make([]byte, 32)
-	if _, err := io.ReadFull(rand.Reader, b); err != nil {
+	if _, err := rand.Read(b); err != nil {
 		return ""
 	}
 	return base64.URLEncoding.EncodeToString(b)