Reduce padding of code blocks

This style tweak reduces the padding of code blocks to make the code
feel less disconnected from the surrounding prose.  (Note that,
effectively, `em` is character height whereas `ch` is character width.)

Author: jonathanhefner

lib/rdoc/generator/template/rails/resources/css/main.css

@@ -518,8 +518,8 @@ html {
 
 .method__source pre,
 .description pre {
-  padding: 0.5em 1em;
-  border-radius: 8px;
+  padding: 0.5ch 1ch;
+  border-radius: 4px;
   overflow-x: auto;
 }