Fix excessive margin on <ol> at end of block

The bottom margin on <ol> was causing some weird spacing when it's the
last element inside a block. This is most noticeable on a blockquote
where the background color extends further down than it should.

tildes/scss/_base.scss

@@ -161,6 +161,10 @@ ol {
     margin-top: 0.2rem;
     max-width: $paragraph-max-width - 2rem;
   }
+
+  &:last-child {
+    margin-bottom: 0.2rem;
+  }
 }
 
 p {