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

[Gavin Bierman]

Updated list of delivered JEPs to reflect that JEP 420 has now been delivered

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

Commit messages:
 - Updated list of delivered JEPs

Changes: https://git.openjdk.java.net/amber-docs/pull/14/files
 Webrev: https://webrevs.openjdk.java.net/?repo=amber-docs&pr=14&range=00
  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/amber-docs/pull/14.diff
  Fetch: git fetch https://git.openjdk.java.net/amber-docs pull/14/head:pull/14

PR: https://git.openjdk.java.net/amber-docs/pull/14