Wrong method signature in doc

The doc for `head` says that the method is:

`head([{'-n', <num>},] file [, file ...])`

But it should be:

`head([{'-n': <num>},] file [, file ...])`

(colon in options object)

**_@ariporad's EDIT:_**

``` diff
- head([{'-n', <num>},] file [, file ...])
+ head([{'-n': <num>},] file [, file ...])
```
