doc: add missing underscore for markdown italics

A closing underscore was missing from the `poll` section of the event
loop doc. Also emboldened a word for continuity and readability.

PR-URL: https://github.com/nodejs/node/pull/6529
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

doc/topics/the-event-loop-timers-and-nexttick.md

@@ -170,7 +170,7 @@ through  its queue of callbacks executing them synchronously until
 either the queue has  been exhausted, or the system-dependent hard limit
 is reached.
 
-* _If the `poll` queue is **empty**, one of two more things will
+* _If the `poll` queue **is empty**_, one of two more things will
 happen:
   * If scripts have been scheduled by `setImmediate()`, the event loop
   will end the `poll` phase and continue to the `check` phase to