RFR: 8291358: Fix the "overridding" typo

[Pavel Rappo]

Please review this trivial grammar fix. I checked that this `@summary` is not a summary of a commit in openjdk/jdk, so no harm in fixing it now:

    * @summary Verify the presence visible members in the case of
    *          member hiding and overriding.

That said, the `@summary` is still a broken sentence, which I'm not trying to fix beyond addressing the typo in question.

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Commit messages:
 - Fix the "overridding" typo

Changes: https://git.openjdk.org/jdk/pull/9660/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9660&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291358
  Stats: 4 lines in 2 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/9660.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9660/head:pull/9660

PR: https://git.openjdk.org/jdk/pull/9660