Fix PHP warning due to a wrong argument given to drush_print_table

A file name is being given as a `$handle` to [drush_print_table()](https://github.com/drush-ops/drush/blob/master/includes/output.inc#L209-L212) instead of a file handler, thus
provoking the following warning after runing drush topic docs-errorcodes:

```
fwrite() expects parameter 1 to be resource, string given output.inc:35
```