improve 20.130 draw:gradient-step-count

http://docs.oasis-open.org/office/v1.2/os/OpenDocument-v1.2-os-part1.html#property-draw_gradient-step-count
The current text is this:
"The draw:gradient-step-count attribute specifies the gradient step count of a color interpolation to be a fixed value. By default, the step count is automatically calculated based on the size and resolution of the filled area.
A gradient step count of color interpretation shall be 3 or greater.
A gradient step count of color interpretation may be above 256."

If you have set a gradient-stop-count to a value in a parent style there exists no way to go back to the behavior, that is described as the default behavior. You cannot omit the attribute to get the default behavior, because then the value at the parent is used because of the inheritance rules. Therefore I suggest to use the value 0 to get the default behavior. This value is allowed by the schema and is used in LibreOffice.

In addition I correct the wording "interpretation". The term "color interpretation" makes no sense here. It should be "color interpolation" as in the first sentence.