RFR: 8319753: Duration javadoc has "period" instead of "duration" in several places

[Eamonn McManus]

This change simply replaces "period" with "duration", or in one case replaces "period unit" with just "unit".

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Commit messages:
 - 8319753: Duration javadoc has "period" instead of "duration" in several places

Changes: https://git.openjdk.org/jdk/pull/16573/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16573&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319753
  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/16573.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16573/head:pull/16573

PR: https://git.openjdk.org/jdk/pull/16573