doc: fix mkdtemp example by removing hyphen

The example uses only `/tmp`, not `/tmp-`. So, the result cannot be
`/tmp-abc123`, but just `/tmpabc123`.

PR-URL: https://github.com/nodejs/node/pull/6834


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>