docs(demo): fix typo in injected variable name

[jtheoof]

The service injected is ValueService, however the name of the variable
does not reflect that. It's actually confusing since it's the name of
the class being created.

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
[ ] Documentation content changes
[x] angular.io application / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Simple typo in the docs of angular.io.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[ngbot]

I see that you just added the PR action: merge label, but the following checks are still failing:
     status "ci/circleci: lint" is failing
     status "continuous-integration/travis-ci/pr" is failing
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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: can you update this pr with commit message: "docs: ..." rather than "docs(demo): ..."

[jtheoof]

sure thing. Should I be worried about the ci failures ? They seem unrelated to my changes.

[jtheoof]

Rebased and fixed commit message.

[IgorMinar]

thanks for the rebase. the ci is now green

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