doc: remove unnecessary bind(this)

PR-URL: #4797
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Chris Dickinson <chris@neversaw.us>
Reviewed-By: Roman Reiss <me@silverwind.io>

doc/api/process.markdown

@@ -627,7 +627,7 @@ but before any I/O has occurred.
 
       process.nextTick(() => {
         this.startDoingStuff();
-      }.bind(this));
+      });
     }
 
     var thing = new MyThing();