[css-color-5] Fix intermediate color in color-mix(red yellow 65%) example.

[dbaron]

The main bug was reported both in a comment in the spec and in
w3ctag/design-reviews#526 (comment)

This also fixes dropping of a "9" digit in the red's chroma midway
through the calculations.

Note that the correct intermediate is well out of sRGB. I just chose to
use it anyway and add a note about such cases.

[dbaron]

Though it occurs to me that my "some of the intermediate colors" note might be wrong because it seems likely to be all of the intermediate colors.

[dbaron]

Though I guess this is inconsistent with the handling of out-of-gamut colors in example 8.

[LeaVerou]

We should either do gamut mapping for --color or not show anything. We definitely don't want to do clipping, which is what browsers do if you throw negative coords at them.
Also, it appears that there is another problem with this example, but I'll open a new issue for that.

[dbaron]

I changed it to an entirely different example that doesn't go out of gamut (by using colors at the endpoints that are both darker and mix sRGB primaries, although in hindsight I may have only needed the latter difference).