RFR: 8341541: Wrong anchor in wrapper classes links

[Hannes Wallnöfer]

Please review a doc change to fix the target anchor for the "wrapper class" links added in #21215. 

In addition to changing the anchor to "wrapperClass" (singular) I also moved the id attribute to the `<p>` tag so that browser view is positioned over the whole paragraph instead of just the term "wrapper classes".

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Commit messages:
 - JDK-8341541: Wrong anchor in wrapper classes links

Changes: https://git.openjdk.org/jdk/pull/21361/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21361&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341541
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21361.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21361/head:pull/21361

PR: https://git.openjdk.org/jdk/pull/21361