mongodbreader.md demo json配置文件错误

https://github.com/alibaba/DataX/blob/master/mongodbreader/doc/mongodbreader.md
配置文件中有拼写错误
```json
{
"name": "frontcat_id",
"type": "Array",
"spliter": ""
}
```
spliter => splitter。 代码和说明文件都是用的splitter

Open mongodbreader/doc/mongodbreader.md and inspect the JSON demo containing the frontcat_id field. Compare the option name with the DataX code and documentation, then verify that the demo uses splitter consistently instead of spliter.

Written by the indexing model from the issue text.