doc: fix minor style issue in code examples

I've noticed that a few of the code examples have an minor
indentation issue with the first line, for example:

https://nodejs.org/api/child_process.html#child_process_child_process
This commit attempt to fix this issue by using the solution provided
provided by silverwind and hiendv.

Fixes: https://github.com/nodejs/node/issues/9381
PR-URL: https://github.com/nodejs/node/pull/9482
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

doc/api_assets/style.css

@@ -274,6 +274,7 @@ pre {
 
 pre > code {
   font-size: .8em;
+  padding: 0;
 }
 
 pre + h3 {