Sure!
First, I don't know if my solution will work, but I'll have you backup everything, so there's no risk involved.
1) Make sure iTunes is not running. You're going to manually edit the library from outside it; don't have iTunes interfere.
2) In the Finder, navigate to "Music:iTunes" inside your home folder. You probably have three items appear in there: a binary Library file with an iTunes file icon, a Music folder containing the whole library hieararchy, and finally, an XML copy of the Library file.
3) Create a folder "Library Backup" in there (in "iTunes", not in the "iTunes Music" subfolder), and copy both library files in there.
4) Remove the binary file. We'll have iTunes will automatically re-create that later. (And you still have a backup anyway.)
5) Open the other file (with the ".xml" extension) in your favorite text editor. TextEdit will work fine.
6) Here's the important bit: do a Search and Replace, searching for "Hip Hop/Rap", and, of course, replacing it with "Hip-Hop/Rap".
Pray that it finds something. If it does, do a Replace All. If it doesn't, however, you can skip the rest and move the backups back to where they were, overwriting your copy of the XML file, because that means my suggestion didn't work.
7) Assuming it found something, and you replaced all occurences, save the file and close your text editor.
8) Open iTunes. Don't worry, your library isn't lost. Go to "File", "Import" and select the XML file you edited.
9) Check if your problem is fixed. If not, rant off in here.