R documentation: Make construction of DMatrix consistent.

* Fix inconsistency of construction of DMatrix.
* Fix missing parameters.

R-package/R/xgb.DMatrix.save.R

@@ -7,8 +7,7 @@
 #'
 #' @examples
 #' data(agaricus.train, package='xgboost')
-#' train <- agaricus.train
-#' dtrain <- xgb.DMatrix(train$data, label=train$label)
+#' dtrain <- with(agaricus.train, xgb.DMatrix(data, label = label))
 #' xgb.DMatrix.save(dtrain, 'xgb.DMatrix.data')
 #' dtrain <- xgb.DMatrix('xgb.DMatrix.data')
 #' if (file.exists('xgb.DMatrix.data')) file.remove('xgb.DMatrix.data')