doc: remove superfluos backticks in process.md

These are likely left over from backporting
https://github.com/nodejs/node/pull/4733.

PR-URL: https://github.com/nodejs/node/pull/7681
Reviewed-By: Myles Borins <myles.borins@gmail.com>

doc/api/process.md

@@ -822,7 +822,6 @@ function MyThing(options) {
     this.startDoingStuff();
   });
 }
-```
 
 var thing = new MyThing();
 thing.getReadyForStuff();