Guides CSS to format code tags in h2 headings

Noticed when reading the edge guides that we have some wonky formatting for the case when we use a method name in an h2 heading.

There's already a fix for h3-level headings, so I copied the font-weight (400) and eyeballed a font-size. 2.25rem looked too much to my eyes, so went with 2rem.

guides/assets/stylesrc/_main.scss

@@ -144,6 +144,11 @@ body.guide {
     a:visited {
       color: $rf-brand;
     } // a, a:link, a:visited
+
+    code {
+      font-size: 2rem;
+      font-weight: 400;
+    }
   } // h2
 
   h3 {