MDL-26546 Workshop: fixed incorrect mapping name in the restore of assessment forms

MDL-26546 Workshop: fixed incorrect mapping name in the restore of assessment forms

There was a regression of 8fa86f and ac0f8f where I changed the mapping
names but did not use the new names in add_related_files() call.

All credit goes to Eloy Lafuente for spotting the problem and giving me
a hint on how to fix it.