fix(docs): fix mention of angular-cli.json file to `angular.json` (#701)
fix(docs): fix mention of angular-cli.json file to `angular.json` (#701)

### Please read and mark the following check list before creating a pull request:

 - [x] I read and followed the [CONTRIBUTING.md](https://github.com/akveo/nebular/blob/master/CONTRIBUTING.md) guide.
 - [x] I read and followed the [New Feature Checklist](https://github.com/akveo/nebular/blob/master/DEV_DOCS.md#new-feature-checklist) guide.
 
 #### Short description of what this resolves:

Angular 6 update documentation
akveo/nebular,akveo/nebular,akveo/nebular,akveo/nebular