Tweak the pdf watchlist entry to improve matching.

Add a slash at the end of the regex entries, to only watch those
directories, so random files with "pdf" in the name do not match.

Change-Id: I75a6f8a139c85e54bd283f379bfbd169ed186691
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1669895
Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671153}