RFR: 8334708: FFM: two javadoc problems

[Hannes Greule]

Addresses two simple problems regarding javadocs in the FFM API.

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Commit messages:
 - fix two javadoc problems in FFM

Changes: https://git.openjdk.org/jdk/pull/19820/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19820&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334708
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/19820.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19820/head:pull/19820

PR: https://git.openjdk.org/jdk/pull/19820