[jdk21u-dev] RFR: 8331331: :tier1 target explanation in doc/testing.md is incorrect

[SendaoYan]

Hi,
  This is clean backport of [JDK-8331331](https://bugs.openjdk.org/browse/JDK-8331331), fixed the incorrect explanation :tier1 target in doc/testing.md. No risk.

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Commit messages:
 - Backport 04271dfe7a262379944e2a2cf83a98a3a1b78a74

Changes: https://git.openjdk.org/jdk21u-dev/pull/551/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=551&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331331
  Stats: 8 lines in 2 files changed: 1 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/551.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/551/head:pull/551

PR: https://git.openjdk.org/jdk21u-dev/pull/551