Correct bad URL in token_counter

[jinchihe]

Features

Fix bug:

2024-03-19 14:52:18.433 | WARNING  | metagpt.provider.openai_api:_calc_usage:250 - usage calculation failed: num_tokens_from_messages() is not implemented for model gpt4. See https://github.com/openai/openai-python/blob/main/chatml.md for information on how messages are converted to tokens.

In fact the link https://github.com/openai/openai-python/blob/main/chatml.md as been removed long time, that mislead users. The PR is correct the link.

With the changes, the output as following:

2024-03-19 15:59:26.193 | WARNING  | metagpt.provider.openai_api:_calc_usage:247 - usage calculation failed: num_tokens_from_messages() is not implemented for model gpt4. See https://cookbook.openai.com/examples/how_to_count_tokens_with_tiktoken for information on how messages are converted to tokens.

Feature Docs

Influence

Result

Other

[geekan]

lgtm