docs(compiler): fix a line about ivy library

[lacolaco]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] angular.io application / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Mistaken

What is the new behavior?

Correct

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

cc/ @chuckjaz

[ngbot]

I see that you just added the PR action: merge label, but the following checks are still failing:
     missing required label: "PR target: *"
     status "continuous-integration/travis-ci/pr" is failing
     status "code-review/pullapprove" is pending

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