[Extensions] Fix a typo of "occurences" -> "occurrences"

Bug: None

Change-Id: I857e8614accc401c358826c2208a8c749fcdab3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1669070
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Tim Judkins <tjudkins@google.com>
Cr-Commit-Position: refs/heads/master@{#671163}