The too minor reason exists because all edits by low rep users have to be reviewed by another user, therefore we try to maximize the benefit of their effort by encouraging only substantial edits. In this case your edit made the question grammatically correct, but the meaning was still clear in the incorrect version, so little was gained. Somewhat more informally the question you edited is a bit older and hasn't had any activity in a while, editing old settled questions with minor typos hurts newer questions by pushing them off the page faster, so a general rule of thumb is to only edit older questions when the edit would be a marked improvement rather than a minor grammatical or spelling error.