DATACASS-516 - Add return type to sample code in reference docs.

Original pull request: #117.

src/main/asciidoc/reference/cassandra.adoc

@@ -421,7 +421,7 @@ public class AppConfig extends AbstractCassandraConfiguration {
   /*
    * Provide a keyspace name to the configuration.
    */
-  public getKeyspaceName() {
+  public String getKeyspaceName() {
     return "mykeyspace";
   }
 }