minor fix of the $interval doc

[chrootsu]

The promise can be undefined
https://github.com/angular/angular.js/blob/v1.4.3/src/ng/interval.js#L180-L188
https://github.com/angular/angular.js/blob/v1.4.3/test/ng/intervalSpec.js#L309-L312

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[chrootsu]

I signed it!

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CLAs look good, thanks!

[caitp]

I suppose this matches the docs for $timeout.cancel(), so lgtm --- the PR text is not super clear, though. The doc isn't saying anything about the type being undefined, it's saying the parameter is optional, and changing the type name. But it's fine.

[chrootsu]

@caitp Thanks for the clarification )