Fix tests

Author: pengrad

library/src/main/java/com/pengrad/telegrambot/model/Audio.java

@@ -97,7 +97,7 @@ public String toString() {
                 ", file_name='" + file_name + '\'' +
                 ", mime_type='" + mime_type + '\'' +
                 ", file_size=" + file_size +
-                ", thumb=" + thumbnail +
+                ", thumbnail=" + thumbnail +
                 '}';
     }
 }