docs: fix typo

[jiangying000]

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?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• [x ] No

Other information

[IgorMinar]

thanks!

[ngbot]

I see that you just added the PR action: merge label, but the following checks are still failing:
     status "ci/circleci: test_docs_examples_0" is 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.

[IgorMinar]

caretaker note: ci failures are unrelated

[angular-automatic-lock-bot]

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