Unify default bottom spacing on margins

Author: NicolaSaunders

assets-src/styles/sass/30-base/_typography.scss

@@ -26,6 +26,7 @@ h1, h2, h3, h4, h5, h6 {
  }
 
  :where(h2, h3, h4, h5, h6) {
+	 margin-block-end: 1em;
 	 margin-block-start: 2em;
  }