change var keyword to const for HEROES constant

[trshafer]

HEROES is a const and is defined as a const originally: https://github.com/angular/angular.io/blob/master/public/docs/_examples/toh-2/ts/app/app.component.ts#L10

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[trshafer]

I signed it!

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CLAs look good, thanks!

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CLAs look good, thanks!

[wardbell]

LGTM