Fix HSTS default expire in ActionDispatch::SSL docs.

[lunks]

I've changed wording a bit to reflect that 180 days is recommended according to #21520 CHANGELOG update, but still make it clear that the minimum is 18 weeks.

[rails-bot]

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[claudiob]

Thanks!

Next time you open a pull request that only affects documentation, make sure to include [ci skip] in your commit message 😁

[lunks]

whoops