hdfsreader.md中 "nullFormat":"\N" 说明有误

hdfsreader中配置nullFormat，如下：
` "nullFormat":"\N"`

报错：
`
经DataX智能分析,该任务最可能的错误原因是:
com.alibaba.datax.common.exception.DataXException: Code:[Common-00], Describe:[您提供的配置文件存在错误信息，请检查您的作业配置 .] - 配置信息错误. 您提供的配置信息不是合法的JSON格式: unclosed string : N . 请按照标准json格式提供配置信息.
at com.alibaba.datax.common.exception.DataXException.asDataXException(DataXException.java:26)
at com.alibaba.datax.common.util.Configuration.(Configuration.java:1066)
at com.alibaba.datax.common.util.Configuration.from(Configuration.java:79)
at com.alibaba.datax.core.util.ConfigParser.parseJobConfig(ConfigParser.java:75)
at com.alibaba.datax.core.util.ConfigParser.parse(ConfigParser.java:26)
at com.alibaba.datax.core.Engine.entry(Engine.java:137)
at com.alibaba.datax.core.Engine.main(Engine.java:204)`

解决：
修改为：
` "nullFormat":"\\N"`

Open hdfsreader.md and locate the nullFormat example. Compare the documented value with the JSON parsing error and the corrected escaped form shown in the issue. Done means the example uses valid JSON escaping and clearly documents the working nullFormat value.

Written by the indexing model from the issue text.