docs: update ivy opt-in flag (#29010)
docs: update ivy opt-in flag (#29010)

Followup to https://github.com/angular/angular/pull/28569#issuecomment-467380883 and https://github.com/angular/angular-cli/pull/13773.

Note: this flag will only work on `@angular/cli@8.0.0-beta.3` (currently unreleased) and above.

PR Close #29010

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