readme勘误

[Weijie Ren]

发现一处 readme 文档错误：

否则会报这个错：

Exception in thread "main" java.lang.NullPointerException
        at org.apache.flink.util.Preconditions.checkNotNull(Preconditions.java:59)
        at com.ververica.cdc.connectors.mysql.source.config.MySqlSourceConfigFactory.createConfig(MySqlSourceConfigFactory.java:313)
        at com.ververica.cdc.connectors.mysql.source.config.MySqlSourceConfigFactory.createConfig(MySqlSourceConfigFactory.java:305)
        at com.ververica.cdc.connectors.mysql.factory.MySqlDataSourceFactory.createDataSource(MySqlDataSourceFactory.java:153)
        at com.ververica.cdc.composer.flink.translator.DataSourceTranslator.translate(DataSourceTranslator.java:54)
        at com.ververica.cdc.composer.flink.FlinkPipelineComposer.compose(FlinkPipelineComposer.java:101)
        at com.ververica.cdc.cli.CliExecutor.run(CliExecutor.java:65)
        at com.ververica.cdc.cli.CliFrontend.main(CliFrontend.java:62)

看了源码才发现，

希望帮到遇到同样问题的朋友。

[Qingsheng Ren]

Considering collaboration with developers around the world, please re-create your issue in English on Apache Jira under project Flink with component tag Flink CDC. Thank you!