max_seq_length is no longer a valid param in SFTTrainer

[yuexu-98]

Here is the PR for this fix: #1170

The issue:

• max_seq_length is no longer a valid param in SFTTrainer, instead we use max_length in SFTConfig
• since this param is not important in this chapter, I delete the outdated line to avoid confusion
• Link: https://huggingface.co/docs/trl/main/en/sft_trainer