Jen,
Currently that option is not available. As you see in the lead's record under the Lead Info tab, the interesting moment fields are not editable and there is noway of editing them in Marketo Sales Insight in SFDC either. With that in mind, it would be a good idea to suggest that enhancement in the Ideas section of Community. The product management team will then take a few things into conisderation and decide whether or not that enhancement will make it into the next release. True suggesting an idea won;t help you now but can you guarantee that there won;t be any more typos in the future? My two cents ..