Replace mention of "cluster" by "session" in DriverConfigLoader.close()

Author: adutra

core/src/main/java/com/datastax/oss/driver/api/core/config/DriverConfigLoader.java

@@ -376,7 +376,7 @@ static DriverConfigLoader compose(
   boolean supportsReloading();
 
   /**
-   * Called when the cluster closes. This is a good time to release any external resource, for
+   * Called when the session closes. This is a good time to release any external resource, for
    * example cancel a scheduled reloading task.
    */
   @Override