Fix rtype formatting in cell.value docstring

Created originally on Bitbucket by adamjstewart (Adam J Stewart)

Was already merged in Bitbucket before import, marked as merged by the import user

Not 100% sure if I'm doing this multi-line rtype properly, but the way it was being done is definitely wrong:

http://openpyxl.readthedocs.io/en/default/api/openpyxl.cell.cell.html#openpyxl.cell.cell.Cell.value