RFR: 8302856: Typo in FlightRecorderMXBeanImpl

[Per Minborg]

This PR fixes two typos.

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Commit messages:
 - Fix typos

Changes: https://git.openjdk.org/jdk/pull/12655/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12655&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302856
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12655.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12655/head:pull/12655

PR: https://git.openjdk.org/jdk/pull/12655