RFR: 8275815: OCA link in README.md and CONTRIBUTING.md is broken

[Kevin Rushforth]

The PR fixes the broken links as described in the JBS issue. The correct link for the Oracle Contributor Agreement (OCA) is:

https://oca.opensource.oracle.com/

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Commit messages:
 - 8275815: OCA link in README.md and CONTRIBUTING.md is broken

Changes: https://git.openjdk.java.net/jfx/pull/652/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=652&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275815
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/652.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/652/head:pull/652

PR: https://git.openjdk.java.net/jfx/pull/652