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

docs/articles/install-into-existing.md

@@ -54,7 +54,7 @@ Same way you can enable Auth Module (more details under [Auth Module Concepts &
 ## Install Styles
 Now, let's import Nebular styles
 
-Include Bootstrap and default Nebular theme CSS files into your `.angular-cli.json` file:
+Include Bootstrap and default Nebular theme CSS files into your `angular.json` file (or `.angular-cli.json` for Angular < 6.0):
 
 ```scss
 "styles": [