Fix accessibility owners in ash/events/OWNERS

Some files were renamed but the OWNERS file didn't get updated, this is
just restoring the property that the accessibility team is also owners
of the accessibility-related event handlers / event rewriters.

Bug: none
Change-Id: Iac759f85fc774570b61bce7adc6947f2cbb6b417
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2505783Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822213}

ash/events/OWNERS

-per-file spoken_feedback_event_rewriter*=file://ui/accessibility/OWNERS
+per-file accessibility_event_rewriter*=file://ui/accessibility/OWNERS
 per-file select_to_speak_event_handler*=file://ui/accessibility/OWNERS
-per-file switch_access_event_handler*=file://ui/accessibility/OWNERS