doc: make param names consistent & fix doc link

The parameter to `fs.mkdtemp` is actually `prefix`, not `template`,
because the string passed is appended with the randomly generated
string.

The doc link for `fs.mkdtemp()` was missing and it is included in this
patch.

PR-URL: https://github.com/nodejs/node/pull/6832


Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: James M Snell <jasnell@gmail.com>