[changeset] clarification to pcolor doc-string

[Francesco Potorti`]

>Francesco, I've entered this changeset in your name. I'm happy to  
>assist in further changes as well. 

Thank you.

>+## @code{shading} modifies an attribute determining the manner by which the
>+## face color of each cell is interpolated from the values of @var{c},
>+## and the visibility of the cells' edges. By default the attribute is
>+## "flat", which renders a single color for each cell's face with the edge
>+## visible.

The text is correct, apart from the fact that this is not the "flat"
attribute, but "interp" or "faceted" (I can not tell which).