docs: fixed path typo in tutorial(Routing, HTTP)

[timxzz]

The path to app.component.css should start from current directory, not app/, since the file is already in app/

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[timxzz]

I signed it!

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[wardbell]

Good catch!