Spinner code example lacks module import in documentation

### Issue type

**I'm submitting a ...**
* [X] bug report
* [ ] feature request

### Issue description
the spinner documentation code examples lack the module import of NbSpinnerModule in app.module.ts

**Current behavior:**
The code examples shows the component's typescript file only
**Expected behavior:**
Have a second tab that displays the import of NbSpinnerModule in app.module.ts

**Steps to reproduce:**
open [spinner module documentation](https://akveo.github.io/nebular/docs/components/spinner/overview#nbspinnerdirective) and see examples