fix: fix openai option gucs setting.

[my-vegetable-has-exploded]

I think those code don't matchs.

let base_url = guc_string_parse(&OPENAI_BASE_URL, "vectors.openai_base");

and

    GucRegistry::define_string_guc(
        "vectors.openai_base_url",
        "The base url of OpenAI or compatible server.",
        "",
        &OPENAI_BASE_URL,
        GucContext::Userset,
        GucFlags::default(),
    );

[cutecutecat]

Thanks for your fix. It's my fault without carefully check.