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>

doc/api/fs.md

@@ -914,7 +914,7 @@ const tmpDir = '/tmp';
 fs.mkdtemp(tmpDir, (err, folder) => {
   if (err) throw err;
   console.log(folder);
-    // Will print something similar to `/tmp-abc123`.
+    // Will print something similar to `/tmpabc123`.
     // Note that a new temporary directory is created
     // at the file system root rather than *within*
     // the /tmp directory.