Update paper-input-decorator.html

[EdmondChuiHW]

Updated the selector to the correctly select floating label color when the input is focused.

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[EdmondChuiHW]

signed CLA

[googlebot]

CLAs look good, thanks!

[addyosmani]

Per https://github.com/Polymer/paper-input/blob/master/paper-input-decorator.css#L66, this does indeed appear to be correct. cc @morethanreal for her take before merge.