RFR: Fix a typo in guide

[Weijun Wang]

`s/RPD2/RDP2/`

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Commit messages:
 - typo

Changes: https://git.openjdk.org/guide/pull/144/files
  Webrev: https://webrevs.openjdk.org/?repo=guide&pr=144&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/guide/pull/144.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/144/head:pull/144

PR: https://git.openjdk.org/guide/pull/144