howto/git: fix headings

All the howtos were under the "Commit hooks" docs, weird.

howto/git.md

@@ -240,13 +240,13 @@ Changes to most repositories are reported to:
 
  * Some repositories have a dedicated mailing list for commits at https://lists.torproject.org
 
-### How to migrate a Git repository from legacy to GitLab?
+## How to migrate a Git repository from legacy to GitLab?
 
 Moving a repository from Gitolite to GitLab proceeds in two parts. One
 part can be done by any user with access to GitLab. The second part
 needs to be done by TPA.
 
-#### User part: importing the repository into GitLab
+### User part: importing the repository into GitLab
 
 This is the part you need to do as a user to move to GitLab:
 
@@ -306,7 +306,7 @@ please make that explicit in the issue and use the proper verb tenses.
 See [issue tpo/tpa/team#41181](https://gitlab.torproject.org/tpo/tpa/team/-/issues/41181) for an example issue as well,
 although that one doesn't use the proper verb tenses
 
-#### TPA part: lock down the repository and add redirections
+### TPA part: lock down the repository and add redirections
 
 This part handles the server side of things. It will install a
 `pre-receive` hook in the Git repository to forbid pushes,
@@ -341,7 +341,7 @@ It assumes the repository has already been migrated.
          
          repo debian/torproject-keyring
 
-### Mirroring a gitolite repository to GitLab
+## Mirroring a gitolite repository to GitLab
 
 This procedure is DEPRECATED. Instead, consider [migrating the
 repository to GitLab permanently](#how-to-migrate-a-git-repository-from-legacy-to-gitlab) or simply destroying the
@@ -383,7 +383,7 @@ This procedure is kept for historical purposes only.
     In that example, the `translation.git` repository will push to the
     `tpo/web/translation` mirror.
 
-### Archiving a repository
+## Archiving a repository
 
 IMPORTANT: this procedure is DEPRECATED. Repositories archived on
 Gitolite *still* will be migrated to GitLab, follow the [migration
@@ -435,7 +435,7 @@ The only downside with that approach is that a git clone will not warn
 about the project redirection, but I am not sure there's a way to fix
 that.
 
-### GitHub and GitLab Mirrors implementation details
+## GitHub and GitLab Mirrors implementation details
 
 Some repositories are mirrored to https://github.com/torproject
 organization and to the https://gitlab.torproject.org/ server, through