RFR: Fix code sample in README

Tue Dec 13 15:15:31 UTC 2022

The Point sample in the README contained a reference to `MemorySession`, which is gone from the 20 API.

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Commit messages:
 - Fix code sample in README

Changes: https://git.openjdk.org/jextract/pull/98/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=98&range=00
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jextract/pull/98.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/98/head:pull/98

PR: https://git.openjdk.org/jextract/pull/98