RFR: 8319166: Typos in the JavaDocs for MemorySegment

[Per Minborg]

This PR proposes to fix a number of JavaDoc typos in the `MemorySegment::getAtIndex` and `::setAtIndex`.

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Commit messages:
 - Fix typos in the JavaDocs for MemorySegment

Changes: https://git.openjdk.org/jdk/pull/16469/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16469&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319166
  Stats: 36 lines in 1 file changed: 0 ins; 0 del; 36 mod
  Patch: https://git.openjdk.org/jdk/pull/16469.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16469/head:pull/16469

PR: https://git.openjdk.org/jdk/pull/16469