RFR: git-pr: use correct argument in git-pr issue

[Erik Helin]

Hi all,

please review this small patch that fixes `git pr issue --remove` to use the correct argument (`--remove`, not `--add`).

Testing:
- Manual testing on Linux x64

Thanks,
Erik

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Commit messages:
 - git-pr: use correct argument in git-pr issue

Changes: https://git.openjdk.java.net/skara/pull/696/files
 Webrev: https://webrevs.openjdk.java.net/skara/696/webrev.00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/696.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/696/head:pull/696

PR: https://git.openjdk.java.net/skara/pull/696