Fix broken samples links for String.Format and DateTime.Parse

[gewarren]

Depends on dotnet/samples#3598.

Contributes to dotnet/docs#17890.

[gewarren]

Preview links:

https://review.docs.microsoft.com/en-us/dotnet/api/system.datetime.parse?view=netframework-4.8&branch=pr-en-us-4243#examples

https://review.docs.microsoft.com/en-us/dotnet/api/system.string.format?view=netframework-4.8&branch=pr-en-us-4243#examples

[IEvangelist]

Looks good to me