ElasticSearch connector's document was not incorrect linked to external repo

In the doc, It still use "flink-version" for flink-connector-elastiacsearch instead of the version in the external repository.

<dependency>
<groupId>org.apache.flink</groupId>
<artifactId>flink-connector-elasticsearch6</artifactId>
<version>1.18-SNAPSHOT</version>
</dependency>