fix typo in the instructions

Hi Jason,

After many years free from bug-reports, it seems I've discovered the sole (insignificant) flaw in this simple but great Vim plugin. :)

As a fan of typographically correct punctuation, I've been happily using UniCycle (great name, BTW) since I discovered it a month ago. It's so much more convenient than having to check the docs to find out the correct digraph.

Regards,
Anthony