RFR: JDK-8212115 : Typo in javadoc for javafx.stage.Window

[Martin Kröning]

Please review the following fix for a typo in the javadocs:

https://bugs.openjdk.java.net/browse/JDK-8212115
https://github.com/javafxports/openjdk-jfx/pull/227

Thanks,
Martin