fix(docs): update function signature of `request_key_builder`

[HurSungYun]

I believe the function signature of request_key_builder in the example is outdated.

This PR updates the example to synchronize with the signature of default_key_builder available at: https://github.com/long2ice/fastapi-cache/blob/91ba6d75524b1f6a17ae6a3300ab51b9aa1fcf71/fastapi_cache/key_builder.py#L8

[HurSungYun]

@long2ice can you please review this PR? thanks in advance