[R] Re-generate Roxygen2 doc (#6915)

R-package/man/xgb.cv.Rd

@@ -160,7 +160,7 @@ Adapted from \url{https://en.wikipedia.org/wiki/Cross-validation_\%28statistics\
 }
 \examples{
 data(agaricus.train, package='xgboost')
-dtrain <- xgb.DMatrix(agaricus.train$data, label = agaricus.train$label)
+dtrain <- with(agaricus.train, xgb.DMatrix(data, label = label))
 cv <- xgb.cv(data = dtrain, nrounds = 3, nthread = 2, nfold = 5, metrics = list("rmse","auc"),
                   max_depth = 3, eta = 1, objective = "binary:logistic")
 print(cv)