Remove superfluous wildcards in paths

We can see from job output such as https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/2633627903 that the .png glob expression works with a single *

I ran a quick test with the amended patterns and my dummy files were identified without issue:

lee@lee-x1:~/gitlab-development-kit/gitlab$ ./unlinked-images.sh doc/operations/metrics
Unlinked file: doc/operations/metrics/img/prometheus_integration_alerts.png
Unlinked file: doc/operations/metrics/img/testabc123.gif
Unlinked file: doc/operations/metrics/img/testabc123.jpg
Unlinked file: doc/operations/metrics/img/testabc123.png