Is it possible to correct the Lead Source Detail from a contact?
Yes, you can go into the lead/contact record in Marketo. Search for that field and then edit it, you should be able to type in whatever you'd like. If you are updating on the mass level you should be able to create a smart campaign with a filter for the Lead Source Detail with the typo and then in the flow change the data value to the correct lead source detail.
and if you need to do it for a whole list of records, you can run larger batches via a smart campaign. Just use the flow step "Change Data Value," and select Lead Source Detail.
Retrieving data ...