Fixed incorrect timed keyframe expression (#3921)

## PR Type
What kind of change does this PR introduce?
<!-- Please uncomment one or more that apply to this PR. -->

- Bugfix
<!-- - Feature -->
<!-- - Code style update (formatting) -->
<!-- - Refactoring (no functional changes, no api changes) -->
<!-- - Build or CI related changes -->
<!-- - Documentation content changes -->
<!-- - Sample app changes -->
<!-- - Other... Please describe: -->


## What is the current behavior?
<!-- Please describe the current behavior that you are modifying, or link to a relevant issue. -->
The `TimedKeyFrame` APIs are using an incorrect expression, resulting in the animation not playing.

## What is the new behavior?
<!-- Describe how was this issue resolved or changed? -->
Fixed the expression to calculate the normalized timestamp of timed keyframes, so they work fine now 😄

## PR Checklist

Please check if your PR fulfills the following requirements:

- [X] Tested code with current [supported SDKs](../readme.md#supported)
- [ ] ~~Pull Request has been submitted to the documentation repository [instructions](..\contributing.md#docs). Link: <!-- docs PR link -->~~
- [ ] ~~Sample in sample app has been added / updated (for bug fixes / features)~~
    - [ ] ~~Icon has been created (if new sample) following the [Thumbnail Style Guide and templates](https://github.com/windows-toolkit/WindowsCommunityToolkit-design-assets)~~
- [X] New major technical changes in the toolkit have or will be added to the [Wiki](https://github.com/windows-toolkit/WindowsCommunityToolkit/wiki) e.g. build changes, source generators, testing infrastructure, sample creation changes, etc...
- [X] Tests for the changes have been added (for bug fixes / features) (if applicable)
- [X] Header has been added to all new source files (run *build/UpdateHeaders.bat*)
- [X] Contains **NO** breaking changes

Author: msftbot[bot]

Microsoft.Toolkit.Uwp.UI.Animations/Builders/TimedKeyFrameAnimationBuilder{T}.cs

@@ -172,7 +172,7 @@ public bool TryInsertExpressionKeyFrame(KeyFrameAnimation animation, TimeSpan du
             [MethodImpl(MethodImplOptions.AggressiveInlining)]
             public float GetNormalizedProgress(TimeSpan duration)
             {
-                return (float)Math.Clamp(this.progress.TotalMilliseconds * 100 / duration.TotalMilliseconds, 0, 1);
+                return (float)Math.Clamp(this.progress.TotalMilliseconds / duration.TotalMilliseconds, 0, 1);
             }
 
             /// <inheritdoc/>