This one you can do within CDpedia. Search for Peak Value 100 % then menu command Edit - > Select All and Album -> Edit. The multiple edit window will come up and Peak Value 100 % should appear in the tags field. Remove it from the tags field and add Peak Value 100%, press okay and it should remove the old tag and add the new one for all the entires.
But should you need to do it in Terminal it would be: