Tokenization Course Issues

Hello,

I believe the corpus and the `word_freqs` output used in the [BPE](https://github.com/huggingface/course/blob/main/chapters/en/chapter6/5.mdx#implementing-bpe) / [WordPiece](https://github.com/huggingface/course/blob/main/chapters/en/chapter6/6.mdx#implementing-wordpiece) implementations have a mismatch simply `Course -> course` is not capitalized in corpus but `word_freqs` seem to use the capitalized version.

To reproduce

```
corpus = [
"This is the Hugging Face course.",
"This chapter is about tokenization.",
"This section shows several tokenizer algorithms.",
"Hopefully, you will be able to understand how they are trained and generate tokens.",
]

tokenizer = AutoTokenizer.from_pretrained("bert-base-cased")

word_freqs = defaultdict(int)
for text in corpus:
words_with_offsets = tokenizer.backend_tokenizer.pre_tokenizer.pre_tokenize_str(text)
words = [word for word, _ in words_with_offsets]
for word in words:
word_freqs[word] += 1

assert word_freqs == defaultdict(int, {'This': 3, 'is': 2, 'the': 1, 'Hugging': 1, 'Face': 1, 'Course': 1, '.': 4, 'chapter': 1, 'about': 1,
'tokenization': 1, 'section': 1, 'shows': 1, 'several': 1, 'tokenizer': 1, 'algorithms': 1, 'Hopefully': 1,
',': 1, 'you': 1, 'will': 1, 'be': 1, 'able': 1, 'to': 1, 'understand': 1, 'how': 1, 'they': 1, 'are': 1,
'trained': 1, 'and': 1, 'generate': 1, 'tokens': 1})
```