doc: better font-smoothing for firefox

does the same thing for firefox as 'antialiased' does for webkit.

PR-URL: https://github.com/iojs/io.js/pull/820
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>

doc/api_assets/style.css

@@ -1,6 +1,7 @@
 /*--------------------- Layout and Typography ----------------------------*/
 html {
   -webkit-font-smoothing: antialiased;
+  -moz-osx-font-smoothing: grayscale;
   -webkit-font-variant-ligatures: none;
           font-variant-ligatures: none;
 }
@@ -465,4 +466,3 @@ td > *:last-child {
     display: none;
   }
 }
-