Replace --apex with --amp in docs (#5037)

references/video_classification/README.md

@@ -18,7 +18,7 @@ We assume the training and validation AVI videos are stored at `/data/kinectics4
 
 Run the training on a single node with 8 GPUs:
 ```bash
-torchrun --nproc_per_node=8 train.py --data-path=/data/kinectics400 --train-dir=train --val-dir=val --batch-size=16 --cache-dataset --sync-bn --apex
+torchrun --nproc_per_node=8 train.py --data-path=/data/kinectics400 --train-dir=train --val-dir=val --batch-size=16 --cache-dataset --sync-bn --amp
 ```
 
 **Note:** all our models were trained on 8 nodes with 8 V100 GPUs each for a total of 64 GPUs. Expected training time for 64 GPUs is 24 hours, depending on the storage solution.