Update with step for manual trigger of initial push mirror

doc/development/omnibus-mirror.md

@@ -50,3 +50,4 @@ The system consists of two groups, one on GitLab.com, and the other on `dev.gitl
    1. In **Git repository URL** enter the path to the `dev.gitlab.org` with `build_mirror_bot@` inserted between `https://` and `dev.gitlab.org`
    1. In **Password** enter the `PAT` from the **Build Team Mirror Bot** entry in the **Build** vault in 1 Password
    1. Leave the remaining options as their default, and click on **Mirror repository**
+   1. Click the **Update Now** button to trigger initial mirroring