small bug/typo in wcsutil.py (too minor for a pull request)

[Thomas Erben]

There is a small bug/typo in the file wcsutil.py, line 597:

It currently is:

x,y = self._findxy_one(lon, lat, xtol=tol)

but it must read:

x,y = self._findxy_one(lon, lat, xtol=xtol)

(tol -> xtol at the end of the line)

[Erin Sheldon]

thanks very much, fixed in master