docs: add missing underline

[musicq]

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?

Missing underline of character 'secondary' on line 104

Issue Number: N/A

What is the new behavior?

Character 'secondary' has been rendered the right way.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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[musicq]

I signed it!

[googlebot]

CLAs look good, thanks!

[gkalpak]

Good catch! Thx 👍

[gkalpak]

The Travis failures are not related to this PR 😕

[ngbot]

I see that you just added the PR action: merge label, but the following checks are still failing:
     status "continuous-integration/travis-ci/pr" is failing

If you want your PR to be merged, it has to pass all the CI checks.

If you can't get the PR to a green state due to flakes or broken master, please try rebasing to master and/or restarting the CI job. If that fails and you believe that the issue is not due to your change, please contact the caretaker and ask for help.

[musicq]

@gkalpak Yep~ Not sure why cannot pass the tests.

[gkalpak]

The tests are probably related to something in master; not your PR.

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