Fix a typo in the command line usage section

Current doc says: _[...]It will generate HTML documentation from all the doc directories it finds[...]_

But in the example we see **docs** (not 'doc') :P

```
$ cd ~/src/yui/yui3
$ selleck
[info] Generating project docs for /Users/rgrove/src/yui/yui3/src/common/docs
[info] Generating component docs for /Users/rgrove/src/yui/yui3/src/autocomplete/docs
[info] Generating component docs for /Users/rgrove/src/yui/yui3/src/history/docs
[info] Done! Docs were written to: ./docs
```

Does it matter though? Or does Selleck look for both (docs and doc)?