Update neutral palette values to match gitlab-ui
Whilst working on a flow I noticed that I was referencing the old neutral palette values.
I've just quickly updated them on the docs to match with what we are using live in gitlab-ui.
$gray-0: #fafafa;
$gray-50: #f0f0f0;
$gray-100: #dbdbdb;
$gray-200: #bfbfbf;
$gray-300: #999;
$gray-400: #868686;
$gray-500: #666;
$gray-600: #5e5e5e;
$gray-700: #525252;
$gray-800: #404040;
$gray-900: #303030;
$gray-950: #1f1f1f;