Fix invalid YAML indentation in mirror workflow

[ehavazli]

Summary

• Non-breaking spaces (U+00A0) had crept into the indentation of .github/workflows/mirror.yml, causing GitHub Actions to reject it as invalid YAML.
• Replaced with regular spaces; file now parses correctly.

Test plan

• Validated with python3 -c "import yaml; yaml.safe_load(open('.github/workflows/mirror.yml'))"

[ehavazli]

@katia-tymofyeyeva @EJFielding
This PR fixes the YAML syntax error (non-breaking spaces in indentation) and updates the destination URL casing, but two remaining CI failures are credential/token issues outside the scope of this fix — they need to be resolved by whoever owns the secrets:

1. NISAR_TOKEN (used in mirror.yml): GitHub rejects the mirror push for every branch with refusing to allow a Personal Access Token to create or update workflow .github/workflows/gitlab_sync.yml without workflow scope. The PAT needs to be regenerated with the workflow scope (classic PAT) or Workflows: Read and write permission (fine-grained PAT), then the NISAR_TOKEN secret updated.

2. GITLAB_TOKEN (used in gitlab_sync.yml): GitLab returns HTTP Basic: Access denied — the token is invalid, expired, or improperly scoped. Needs a fresh GitLab personal access token with write_repository scope, with GITLAB_USERNAME/GITLAB_URL secrets confirmed to match that token's account.

[ehavazli]

Update: restricting the mirror to `main` (last commit) stops the 11 feature-branch rejections, but `main` itself still fails with the same error:

```
! [remote rejected] main -> main (refusing to allow a Personal Access Token to create or update workflow .github/workflows/gitlab_sync.yml without workflow scope)
```

This confirms it's purely the `NISAR_TOKEN` scope problem noted above — `main`'s copy of `.github/workflows/gitlab_sync.yml` differs from the destination's, and GitHub blocks the push until the token has `workflow` scope (classic PAT) or `Workflows: Read and write` (fine-grained PAT). No further code change in this repo will resolve it; the secret needs to be regenerated with that scope.