Fix paragraphs with reversed-out text

Since these two paragraphs sit on a blue background, they should have
white text. But this was getting overridden when the `govuk-body` class
was added in a previous commit, and the paragraphs appeared with black
text.

Previously these paragraphs were inheriting their colour from a parent
element. But a class applied directly to the element is more specific.
So this commit fixes the problem by being more specific again, by
applying the colour to the element, in the context of it’s parent’s
class.

app/assets/stylesheets/components/banner.scss

@@ -82,6 +82,7 @@
 
     margin-top: 0;
     margin-bottom: govuk-spacing(6);
+    color: $white;
 
     &:last-child {
       margin-bottom: 0;

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

@@ -36,6 +36,7 @@ $button-shadow-size: $govuk-border-width-form-element;
 
     p {
       @include core-24;
+      color: $white;
       margin: govuk-spacing(3) 0 govuk-spacing(6);
     }