fix(list): tweak list header color to match spec

Author: brandyscarney

src/components/list/list.md.scss

@@ -46,7 +46,7 @@ $list-md-header-border-top:      1px solid $list-md-border-color !default;
 $list-md-header-font-size:       1.4rem !default;
 
 /// @prop - Text color of the header in a list
-$list-md-header-color:           #858585 !default;
+$list-md-header-color:           #757575 !default;
 
 
 // Material Design Default List