[wav2vec2] Add missing --validation_split_percentage data arg
# What does this PR do?

This PR adds the missing `--validation_split_percentage` arg in wav2vec2 examples.
Default value comes from [`run_wav2vec2_pretraining_no_trainer.py`](https://github.com/huggingface/transformers/blob/70f186f61ebff4ad87fd7cc0fc1e0e0660b485ea/examples/pytorch/speech-pretraining/run_wav2vec2_pretraining_no_trainer.py#L95).

@stas00, @patrickvonplaten