doc: minor fixes event-loop-timers-and-nexttick.md

Minor fixes and enhancements to event-loop-timers-and-nexttick.md
Added missing "be"
Added a link to REPL docs
Added definition of libuv and a link

PR-URL: https://github.com/nodejs/node/pull/9126


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>