[amber-docs] RFR: Updated list of delivered JEPs

[Gavin Bierman]

JEP 420 has been delivered

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Commit messages:
 - Updated list of delivered JEPs

Changes: https://git.openjdk.java.net/amber-docs/pull/13/files
 Webrev: https://webrevs.openjdk.java.net/?repo=amber-docs&pr=13&range=00
  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/amber-docs/pull/13.diff
  Fetch: git fetch https://git.openjdk.java.net/amber-docs pull/13/head:pull/13

PR: https://git.openjdk.java.net/amber-docs/pull/13