Hi Orestes,
As far as I know, unlike the Quiz activity, there is no way to regrade the Lesson activity. If you know exactly what you are doing and are at ease with SQL requests you might try to change the students' marks directly in your moodle site's database, but that is really risky.
Another possibility would be to go to the gradebook and "give" a bonus mark for that Lesson activity, regardless of whether the students got that MCQ question right or wrong. That way, it would be possible for some students to achieve a 100% score.
Joseph