[css-color-5] fix conflicting prose about what the percentage means in color-mix()

Related to w3c#6064

Author: LeaVerou

css-color-5/Overview.bs

@@ -135,7 +135,8 @@ If the specified colorspace has a smaller gamut
 than the one in which the color to be adjusted is specified,
 gamut mapping will occur.
 3. Colors are then <a href="../css-color-4/#interpolation">interpolated in the specified colorspace</a>.
-The result of mixing is the color at the specified percentage along the progression.
+The result of mixing is the color at the specified percentage along the progression
+of the second color to the first color.
 
 Note: As a corrollary, 0% just returns the second color converted to the specified colorspace,
 100% returns the first color converted to the specified colorspace,