doc: add spaces to child.kill example

PR-URL: #1503
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>

Author: enaqx

doc/api/child_process.markdown

@@ -192,7 +192,7 @@ be sent `'SIGTERM'`. See `signal(7)` for a list of available signals.
         grep  = spawn('grep', ['ssh']);
 
     grep.on('close', function (code, signal) {
-      console.log('child process terminated due to receipt of signal '+signal);
+      console.log('child process terminated due to receipt of signal ' + signal);
     });
 
     // send SIGHUP to process