fix(CSS): correct placeholder color

Author: Ryan A. Johnson

src/helix-ui/styles/mixins.less

@@ -1,5 +1,5 @@
 .u-placeholder() {
-  color: @gray-700; // contrast @ 4.6:1
+  color: @gray-750;
   font-style: italic;
   font-weight: 400;
   opacity: 1;