RFR: 8285725: Wrong link to JBS in README.md

[Nir Lisker]

Updated the README link to match the CONTRIBUTING link.

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Commit messages:
 - Fixed link

Changes: https://git.openjdk.java.net/jfx/pull/788/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=788&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285725
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/788.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/788/head:pull/788

PR: https://git.openjdk.java.net/jfx/pull/788