fix(angular): ng-add schematic

[SchnWalter]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

The README.md instructs the user to link the invalid directory.
The npx schematics @ionic/angular:ng-add command generates an invalid angular.json. See my comment on the: 7af0198#r36943372

Issue Number: N/A

What is the new behavior?

• The README.md instructs the user to link the ionic/angular/dist directory.
• The npx schematics @ionic/angular:ng-add generates a valid angular.json file.

Does this introduce a breaking change?

• Yes
• No

cc: @mhartington