Keyword argument 'evaluation_strategy' in TrainingArguments has been replaced with 'eval_strategy'

Hi team,

In Chapter 7: Fine-Tuning a Masked Language Model, the example code uses:
```
TrainingArguments(
...,
evaluation_strategy="epoch",
...
)
```
However, the evaluation_strategy keyword argument in TrainingArguments has now been replaced with eval_strategy. Using the old argument causes an error:
```
TypeError: TrainingArguments.__init__() got an unexpected keyword argument 'evaluation_strategy'
```
See this related discussion for reference:
https://github.com/huggingface/transformers/issues/7974#issuecomment-2803578764

Proposed fix:
Update the code example to use `eval_strategy` instead of `evaluation_strategy` to match the latest transformers library version.