docs(tutorial/step_14): replace broken web platform docs link

Replace broken [webplatform-animations][1] link with [mdn-animations][2].

The original link returns a 404. The closest match that works is
https://webplatform.github.io/docs/css/properties/animation. However,
the notice at the top of the page reads:

> The WebPlatform project, supported by various stewards between 2012
> and 2015, has been discontinued.

The CSS animations guide on MDN web docs is not only current, but also
more comprehensive.

[1]: https://docs.webplatform.org/wiki/css/properties/animations
[2]: https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Animations/Using_CSS_animations

Closes #16294

Author: sagirk

docs/content/tutorial/step_14.ngdoc

@@ -328,7 +328,7 @@ The applied CSS classes are much the same as with `ngRepeat`. Each time a new pa
 ensures that all views are contained within a single HTML element, which allows for easy animation
 control.
 
-For more on CSS animations, see the [Web Platform documentation][webplatform-animations].
+For more on CSS animations, see the [MDN web docs][mdn-animations].
 
 
 ## Animating `ngClass` with JavaScript
@@ -561,4 +561,4 @@ There you have it! We have created a web application in a relatively short amoun
 [caniuse-css-transitions]: http://caniuse.com/#feat=css-transitions
 [jquery]: https://jquery.com/
 [jquery-animate]: https://api.jquery.com/animate/
-[webplatform-animations]: https://docs.webplatform.org/wiki/css/properties/animations
+[mdn-animations]: https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Animations/Using_CSS_animations