assistant: add Jeroen to owner

Bug: None
Change-Id: Iaf4ad87d59a6f957c5aef6623e7c64f72d92c364
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2080195Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745565}

chromeos/assistant/OWNERS

+jeroendh@chromium.org
 updowndota@chromium.org
 wutao@chromium.org
 xiaohuic@chromium.org