RFR: Remove --source from help message

[Jorn Vernee]

This last reference to the `--source` option was missed when https://github.com/openjdk/jextract/pull/186 was fixed.

Removes `--source` from the help message.

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Commit messages:
 - remove --source from help message

Changes: https://git.openjdk.org/jextract/pull/226/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=226&range=00
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jextract/pull/226.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/226/head:pull/226

PR: https://git.openjdk.org/jextract/pull/226