RFR: 8254100: FX: Update copyright year in docs, readme files to 2021

[Ambarish Rapte]

Copyright year in these 3 docs needs to be updated to 2021.

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

Commit messages:
 - doc copyright 2021

Changes: https://git.openjdk.java.net/jfx/pull/330/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=330&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254100
  Stats: 3 lines in 3 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/330.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/330/head:pull/330

PR: https://git.openjdk.java.net/jfx/pull/330