Stepper playground docs use disabled nbStepperNext for disabled previous step

### Issue type

**I'm submitting a ...** (check one with "x")

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
The playground examples contain uses of
```
prev
```

**Expected behavior:**
This code should read as:

```
prev
```

**Steps to reproduce:**
N/A

**Related code:**

https://github.com/akveo/nebular/blob/4c4d8460cde1f3223d402c8d2344efa79fb6998b/src/playground/with-layout/stepper/stepper-vertical.component.html#L17
https://github.com/akveo/nebular/blob/4c4d8460cde1f3223d402c8d2344efa79fb6998b/src/playground/with-layout/stepper/stepper-showcase.component.html#L12
https://github.com/akveo/nebular/blob/4c4d8460cde1f3223d402c8d2344efa79fb6998b/src/playground/with-layout/stepper/stepper-step-change-event.component.html#L14

### Other information:

**npm, node, OS, Browser**
```

```

**Angular, Nebular**
```

```