doc: fixes grammar in timers/tls

This commit fixes a few grammar issues located
within the doc files for timers and tls.

They primarily include incorrect use of a / an
and a single insertion of a comma.

same as [this PR](https://github.com/joyent/node/pull/8581)

Reviewed-By: Fedor Indutny <fedor@indutny.com>
PR-URL: https://github.com/iojs/io.js/pull/76