yapf doesn't recognize "SPACES_AROUND_SUBSCRIPT_COLON" from docs

I found that yapf doesn't recognize the `SPACES_AROUND_SUBSCRIPT_COLON` knob that's listed in the docs. Here's how I produced the error:

#### Config (`setup.cfg`)
```
[yapf]
based_on_style = pep8
spaces_around_subscript_colon = true
```

#### Shell
```
>> yapf -i
yapf: Unknown style option "SPACES_AROUND_SUBSCRIPT_COLON"
```