doc: update description of fs.exists in fs.markdown

per: https://github.com/joyent/node/pull/7944

originally submitted by @oconnore

Minor update to the description in `fs.exists()`

Reviewed By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/2378