PPL guide code snippet minor fix
# What does this PR do?

Minor fix to the code snippet in the [perplexity guide](https://huggingface.co/transformers/perplexity.html), as discussed in [this thread](https://discuss.huggingface.co/t/guide-the-best-way-to-calculate-the-perplexity-of-fixed-length-models/193).

Previously the snippet didn't take into account the length of the the last loop over the data, which can be shorter than the specified `stride` length.