[foreign-memaccess+abi] RFR: 8294871: The spec on the method MemorySegment::segmentOffset() should be corrected

[Per Minborg]

This PR fixes the snippet in `MemorySegment::segmentOffset`.

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Commit messages:
 - Fix snipet in MemorySegment::segmentOffset

Changes: https://git.openjdk.org/panama-foreign/pull/749/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=749&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294871
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/749.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/749/head:pull/749

PR: https://git.openjdk.org/panama-foreign/pull/749