RFR: JDK-8309958: Incorrect @since tag format in Container.java

[Harshitha Onkar]

Minor Javadoc change for since tag in Container.java

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Commit messages:
 - minor java doc change

Changes: https://git.openjdk.org/jdk/pull/14474/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14474&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309958
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14474.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14474/head:pull/14474

PR: https://git.openjdk.org/jdk/pull/14474