message: correct `method_name` in `root_group`
diff: @@ -430,7 +430,7 @@ impl Dataset {
  unsafe {
  let c_group = gdal_sys::GDALDatasetGetRootGroup(self.c_dataset());
  if c_group.is_null() {
  - return Err(_last_null_pointer_err("GDALGetRasterBand"));
  + return Err(_last_null_pointer_err("GDALDatasetGetRootGroup"));
  }
  Ok(Group::from_c_group(self, c_group))
  }
  