fix md file to avoid evaluation crash
# What does this PR do?

Fix the crash due to the memory usage in the instructions for model evaluation in `FINE_TUNE_XLSR_WAV2VEC2.md`.
The original version `test_dataset["speech"][:2]` load the whole speech array into memory which is too large.
Change it to `test_dataset[:2]["speech"]` runs smoothly and much faster.

## Before submitting
- [ ] This PR improves the docs

## Who can review?

@patrickvonplaten 

@patrickvonplaten It seems the finetune notebook is on your own github repository, not on HuggingFace's transformers.<|||||>@ydshieh yes this is correct - if it's ok for you feel free to open a PR there :-) 