[css-color-5] color-mix example 1 should use something other than 50%

[smfr]

https://drafts.csswg.org/css-color-5/#colormix

Example 1 would be clearer with, say lightness(40%), so that step 3 becomes less ambiguous:

mix lightness is 40.083 * 0.4 + 70% * (1 - 0.4) = ...

[svgeesus]

Agreed, will fix.