RFR: 8283869: Update attribution in webkit.md file

[Kevin Rushforth]

Doc-only change to update the license attribution in our `webkit.md` file.

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Commit messages:
 - 8283869: Update attribution in webkit.md file

Changes: https://git.openjdk.java.net/jfx/pull/793/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=793&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283869
  Stats: 5484 lines in 1 file changed: 5483 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/793.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/793/head:pull/793

PR: https://git.openjdk.java.net/jfx/pull/793