Fix minor grammatical problem

"check out" makes more sense here.

Checkout has the following meaning which I don't think we are going for:
```
check·out  
/ˈCHekˌout/
Noun
* A point at which goods are paid for in a supermarket or other store.
* The administrative procedure followed when a guest leaves a hotel at the end of a stay.
```

Whereas check out:

```
check out  
check: examine so as to determine accuracy, quality, or condition; "check the brakes"; "Check out the engine".
```

(using Google's `define:` search)