The typo has been corrected on the official Vatican site:
Please also see my new post, in which I prove that the Pope himself did not pronounce the typo that was, for a time, posted on the Vatican Website.
Below is my original post from the morning of the announcement:
I wish His Holiness Benedict XVI health and many years of the prayerful retirement he has announced.
At least of this posting, the Latin text of his official resignation contains a typo!
Here's an example of the correct formula from a prior Papal document:
I am certain His Holiness knows Latin abundantly better than I and that this is a simple typo and not a grammatical error. He is probably not even personally responsible for the typo. Even so, let's pray it is corrected on the official site.
It is otherwise a grammatically lovely sentence with a Purpose Clause. The actual resignation is conveyed via an Indirect Statement construction:
declaro...me...renuntiare (I declare [that] I am retiring).