docs(validators): Fix typo in code format
docs(validators): Fix typo in code format

Author: Benjamin Dopplinger <b.dopplinger@gmail.com>

Closes https://github.com/dart-lang/angular2/pull/53

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matanlurey/angular,kevmoo/angular,chalin/angular2,chalin/angular2,angulardart/angular,matanlurey/angular,angulardart/angular,angulardart/angular,matanlurey/angular,chalin/angular2,kevmoo/angular,kevmoo/angular