Update IppValidator endpoint checking

Found out that CUPS identifies its printer endpoints as
'/printers/{printer_id}'; updated our validation to match.

Bug: chromium:945409
Test: unittests still pass
Change-Id: Ia40a56c2cf7db8406e7d0cce5f5665f32fa46aed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1686212
Commit-Queue: Luum Habtemariam <luum@chromium.org>
Reviewed-by: Sean Kau <skau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675885}