Update download.sh

fix invalid dataset url

fluid/PaddleCV/video_classification/data/download.sh

 # Download the dataset
 echo "Downloading..."
-wget http://crcv.ucf.edu/data/UCF101/UCF101.rar
-wget http://crcv.ucf.edu/data/UCF101/UCF101TrainTestSplits-RecognitionTask.zip
+wget https://www.crcv.ucf.edu/datasets/human-actions/ucf101/UCF101.rar
+wget https://www.crcv.ucf.edu/wp-content/uploads/2019/03/UCF101TrainTestSplits-RecognitionTask.zip
 
 # Extract the data.
 echo "Extracting..."