RFR: 8340680: Fix typos in javax.lang.model.SourceVersion

[Pavel Rappo]

This PR fixes a few overlooked typos in the recent PR #20937. In particular, it fixes grammar issues which @dansmithcode pointed to: https://github.com/openjdk/jdk/pull/20937#pullrequestreview-2293456337. (#20937 fixed only some of those.)

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/21140/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21140&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340680
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21140.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21140/head:pull/21140

PR: https://git.openjdk.org/jdk/pull/21140