Fix docs for AS::Notifications::Event#duration

Stems from comments by @zzak on e07f3dd:

* https://github.com/rails/docrails/commit/e07f3ddcac394d2a8dc23fc571318b7e8c2497b1#commitcomment-9015634
* https://github.com/rails/docrails/commit/e07f3ddcac394d2a8dc23fc571318b7e8c2497b1#commitcomment-9015639

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