doc: improve child_process.markdown wording

PR-URL: https://github.com/nodejs/node/pull/4138
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/child_process.markdown

@@ -26,9 +26,9 @@ Child processes always have three streams associated with them. `child.stdin`,
 streams of the parent process, or they may be separate stream objects
 which can be piped to and from.
 
-The ChildProcess class is not intended to be used directly.  Use the
+The `ChildProcess` class is not intended to be used directly.  Use the
-[`spawn()`][], [`exec()`][], [`execFile()`][], or [`fork()`][] methods to create a Child
+[`spawn()`][], [`exec()`][], [`execFile()`][], or [`fork()`][] methods to create
-Process instance.
+an instance of `ChildProcess`.
 
 ### Event: 'close'