Fix button shadow colour on product page

The button was inheriting a green box shadow from the default GOV.UK
button style, which is typically used on a white background. On the blue
background, a darker blue shadow looks better I think.

The 15% darkening is the same as used by the default green button:
a3fe44e00e/stylesheets/design-patterns/_buttons.scss (L36)

app/assets/stylesheets/views/product-page.scss

@@ -36,6 +36,7 @@
 
     .button {
 
+      @include box-shadow(0 2px 0 darken($govuk-blue, 15%));
       background: $white;
       margin-right: 15px;