[jfx19] RFR: 8290393: Fix code sample in docs of ObservableValue#flatMap

[John Hendrikx]

Fix small error in flatmap docs

-------------

Commit messages:
 - Fix code sample in docs of ObservableValue#flatMap

Changes: https://git.openjdk.org/jfx/pull/831/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=831&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290393
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/831.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/831/head:pull/831

PR: https://git.openjdk.org/jfx/pull/831