DOC: Correct Documentation for FFTPadImageFilter::SizeGreatestPrimeFactor

PR Checklist

[ ] No API changes were made (or the changes have been approved)
[ ] No major design changes were made (or the changes have been approved)
[ ] Added test (or behavior not changed)
[ ] Updated API documentation (or API not changed)
[ ] Added license to new files (if any)
[ ] Added Python wrapping to new files (if any) as described in ITK Software Guide Section 9.5
[ ] Added ITK examples for all new major features (if any)

Refer to the ITK Software Guide for further development details if necessary.