RFR: git-trees: fix usage message

[Erik Helin]

Hi all,

please review this small patch that fixes the usage message for `git-trees`.

Testing:
- [x] Manual testing on Linux x64

Thanks,
Erik

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Commit messages:
 - git-trees: fix usage message

Changes: https://git.openjdk.java.net/skara/pull/704/files
 Webrev: https://webrevs.openjdk.java.net/skara/704/webrev.00
  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/704.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/704/head:pull/704

PR: https://git.openjdk.java.net/skara/pull/704