Make spark-etl doc clearer about replacing generic types with specific types #1641

bject GeoTrellisETL {
  def main(args: Array[String]): Unit = {
    implicit val sc = SparkUtils.createSparkContext("GeoTrellis ETL", new SparkConf(true))

    try {
      EtlConf(args) foreach { conf =>
        /* parse command line arguments */
        val etl = Etl(conf, modules)
        /* load source tiles using input module specified */
        val sourceTiles = etl.load[I, V]
        /* perform the reprojection and mosaicing step to fit tiles to LayoutScheme specified */
        val (zoom, tiled) = etl.tile(sourceTiles)
        /* save and optionally pyramid the mosaiced layer */
        etl.save[K, V](LayerId(etl.input.name, zoom), tiled)
      }
    } finally {
      sc.stop()
    }
  }
}