Updated groovy docs to point to the new groovy repo

docs/groovy-api/index.asciidoc

@@ -30,7 +30,7 @@ For example, you can define the latest version in your `pom.xml` file:
 --------------------------------------------------
 <dependency>
     <groupId>org.elasticsearch</groupId>
-    <artifactId>elasticsearch-client-groovy</artifactId>
+    <artifactId>elasticsearch-lang-groovy</artifactId>
     <version>${es.version}</version>
 </dependency>
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