Update bug number of TODO in policy_watcher_browser_agent_unittest.mm

This TODO was added here, but it seems to have used the wrong bug
number.
https://chromium-review.googlesource.com/c/chromium/src/+/6180164/21/ios/chrome/browser/policy/model/policy_watcher_browser_agent_unittest.mm@383

Bug: None
Change-Id: I6d4b2aac6513ea98952c003d3fb0ec532a1c1cca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6400600
Reviewed-by: Salma Elmahallawy <esalma@google.com>
Commit-Queue: Vincent Boisselle <vincb@google.com>
Reviewed-by: Vincent Boisselle <vincb@google.com>
Auto-Submit: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/main@{#1439001}