docs(h1): point max_buf_size keep-in-sync note at the real Builder path

[ump45nose]

What

src/proto/h1/io.rs carries a keep-in-sync note above DEFAULT_MAX_BUFFER_SIZE:

Note: if this changes, update server::conn::Http::max_buf_size docs.

There is no server::conn::Http type anymore. The server::conn module exposes only http1 and http2, and the doc that actually states this default (/// Default is ~400kb.) is Builder::max_buf_size in src/server/conn/http1.rs. The 1.x refactor renamed the type but left this pointer behind, so it now directs maintainers to a path that does not exist.

I only updated the path in the note; the constant and its value are unchanged.

Why it matters

The stale pointer makes the "update this doc when you change the default" instruction unactionable — following it leads nowhere. Pointing at server::conn::http1::Builder::max_buf_size restores the intended link.

Test plan

Comment-only change; no code paths touched. Verified at the current master that src/server/conn/mod.rs declares only pub mod http1; / pub mod http2;, and that Builder::max_buf_size with the ~400kb default lives in src/server/conn/http1.rs.