doc: fix typos in timers topic to aid readability

Typos in the `setTimeout` vs. `setImmediate` section were hindering
readability. Fixed these typos.

PR-URL: https://github.com/nodejs/node/pull/6916


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>