AVRO-2551:Changed datum API to value API in examples\quickstop

[zeshuai007]

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[zeshuai007]

@dcreager Do you have a spare moment to review this?

[dcreager]

👍 It's been quite awhile since I've looked at this API, but broadly speaking it looks right.

It also looks like this file is run as a test, too, so if tests are still passing that's a good indication that you got the migration right!

[zeshuai007]

@Fokko Can this PR be merged?