doc: fix color of linked code blocks

Links nested in backticks were not visually distinguishable before, this
fixes them.

PR-URL: https://github.com/nodejs/node/pull/4068
Reviewed-By: Roman Reiss <me@silverwind.io>

doc/api_assets/style.css

@@ -343,6 +343,10 @@ li code {
   padding: 0.2em 0.4em;
 }
 
+a code {
+  color: inherit;
+}
+
 span.type {
   color: #222;
 }