doc: fix sentence grammar timers.markdown

Just added ', it' because the phrasing did not seem correct.

PR-URL: #815
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

doc/api/timers.markdown

@@ -34,7 +34,7 @@ Stops an interval from triggering.
 
 The opaque value returned by `setTimeout` and `setInterval` also has the method
 `timer.unref()` which will allow you to create a timer that is active but if
-it is the only item left in the event loop won't keep the program running.
+it is the only item left in the event loop, it won't keep the program running.
 If the timer is already `unref`d calling `unref` again will have no effect.
 
 In the case of `setTimeout` when you `unref` you create a separate timer that