Increase contrast for comments in code samples

Author: adamwathan

docs/source/_assets/less/prism.less

@@ -73,7 +73,7 @@ pre[class*="language-"] {
 .token.prolog,
 .token.doctype,
 .token.cdata {
-  @apply .text-grey;
+  @apply .text-grey-dark;
 }
 
 .token.punctuation {