fix Polynomial.td doc filename (#88900)

Not sure how best to test this, but I think it fixes the error
https://github.com/llvm/mlir-www/actions/runs/8699908058/job/23859264085#step:7:1111



Co-authored-by: Jeremy Kun <j2kun@users.noreply.github.com>
Co-authored-by: Jacques Pienaar <jpienaar@google.com>