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>
2 jobs for readme-fix in 51 seconds (queued for 1 second)
Status Job ID Name Coverage
  Prep
passed #2736872
container-prep

00:00:26

 
  Test
passed #2736873
test

00:00:25