Update Less Mixin section for Gradients

Created by: keeneyemedia

It seems the current documentation to create a 3-color gradient creates an error with the color stop being declared as a decimal. Ex:

gradient > .vertical-three-colors(#777; #333 (closed); .25; #000);

Should be a percentage. Ex:

gradient > .vertical-three-colors(#777; #333 (closed); 25%; #000);