Fix minor error

- Typo in code was corrected by aeeea56cb16fa38953d2567173e8b5ca9caf7338 (https://github.com/OfflineIMAP/offlineimap/commit/aeeea56cb16fa38953d2567173e8b5ca9caf7338#diff-78d7da517b1cde119096e12b3b592fa9R125)
- Update prose to match code