Revert change from % to ^

I think my change of the spec from % to ^ was premature,
partially because I now mildly prefer % again,
but mostly because it’s what we presented to plenary
on August and because it’s what Babel currently supports.

This is a tentative change and has no bearing on what the actual final topic token will be.

As per #91 (comment).

See also #91 (comment),
which I think was now premature.