Typo: convertable -> convertible

This is not strictly correct though because array subclasses such as masked arrays are allowed, they are not converted with `numpy.asarray()`. The following might be more correct:

"If the object contains phenomenon values it can be a numpy array or array subclass, or any object that is convertible to an array using :func:`numpy.asarray`."

although I'm not sure if that is more detail than is needed. Perhaps some one else has thoughts on this.