fix typo in cell test

Created originally on Bitbucket by tfussell (Thomas Fussell)

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

This is really minor, but it seemed like a good chance to make my first contribution to openpyxl. I messed up the previous pull request since this is my first time using Mercurial. Sorry for the spam. I hope this is the correct branch.