fix(CSS): correct space below only-child headings

* in cases where a heading is an only child, white space should not be
  added below it

Author: Ryan A. Johnson

src/helix-ui/styles/modules/typography.less

@@ -3,7 +3,11 @@ h1, h2, h3, h4, h5, h6 {
   font-family: @font-sans-serif;
   font-weight: 400;
   line-height: 1.5;
-  margin: 0 0 0.5rem;
+  margin: 0;
+
+  & + * {
+    margin-top: 0.5rem;
+  }
 }
 
 // DEFINE