fix changelog release date

[Coeur]

The correct release date for beta 5 is 29th, as seen here:

• 2e714f9
• 5.0.0-beta.4...5.0.0-beta.5

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] angular.io application / infrastructure changes
[ ] Other... Please describe:

Does this PR introduce a breaking change?

[ ] Yes
[x] No

[vicb]

Please fix the commit message format. Thanks

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