README.md: fix "issues" link

GitLab links without the `/-/` between the project and page within the project
are not going to be valid anymore once GitLab is updated to the next version.

See:
https://about.gitlab.com/releases/2020/05/06/gitlab-com-13-0-breaking-changes/#removal-of-deprecated-project-paths

Signed-off-by: Eric Engestrom <eric@engestrom.ch>
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