Minor correction for default argument in ellipsoid.m

[John W. Eaton]

On 24-Oct-2008, Søren Hauberg wrote:

| Hi,
|   The 'ellipsoid' function accepts 6 or 7 input arguments. If the 7th
| argument is missing, it defaults to 20. This doesn't work due to a
| trivial error, that the attached patch fixes.

I made this change.

Thanks,

jwe