RFR: 8260561: [doc] HexFormat has incorrect @since tag

[Roger Riggs]

A doc-only change to correct the @ since tag.

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Commit messages:
 - 8260561: [doc] HexFormat has incorrect @since tag

Changes: https://git.openjdk.java.net/jdk/pull/2270/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2270&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260561
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2270.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2270/head:pull/2270

PR: https://git.openjdk.java.net/jdk/pull/2270