doc: fix <code> inside stability boxes

Fixes: https://github.com/nodejs/node/issues/9714
PR-URL: https://github.com/nodejs/node/pull/9723
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api_assets/style.css

@@ -362,6 +362,11 @@ tt, code {
   padding: .1em .3em;
 }
 
+.api_stability code {
+  background: rgba(0, 0, 0, .1);
+  padding: .1em .3em;
+}
+
 a code {
   color: inherit;
   background: inherit;