`HTMLHeaderTextSplitter` 在初始化时对其已支持的非标题标签抛出 `ValueError`

**What I expect.** `HTMLHeaderTextSplitter` documents `headers_to_split_on` as "a list of `(header_tag, header_name)` pairs representing the headers that define splitting boundaries", with no restriction to `h1` through `h6`. Its splitting engine goes further and handles other tags deliberately, so constructing the splitter with one should work.