RFR: 8324797: Code example in JavaDoc of ObservableValue#when doesn't compile

[Philippe Altherr]

8324797: Code example in JavaDoc of ObservableValue#when doesn't compile

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Commit messages:
 - Fix type of variables in code example from ObservableValue to Property.

Changes: https://git.openjdk.org/jfx/pull/1325/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1325&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324797
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1325.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1325/head:pull/1325

PR: https://git.openjdk.org/jfx/pull/1325