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>