Fixed typo when using a block mixin

So I had a go at using block mixins and doing this didn't work:
```
large()
  @media only screen and (min-width: 992px)
    {block}

+large
  padding: 0
```

It just compiled to `large{padding:0}`, so I did this and it worked:
```
large()
  @media only screen and (min-width: 992px)
    {block}

+large()
  padding: 0
```

Hence I thought I'd update the docs typo :)