Fix gitlab-docs nanoc command path in Procfile

What does this Merge Request do and why?

After recent gdk update I've started getting errors for gitlab-docs and it wouldn't start:

I have it enabled in gdk.yml, as described in https://gitlab.com/gitlab-org/gitlab-development-kit/blob/main/doc/howto/gitlab_docs.md#enable-gitlab-docs .

It seems like in MR !2504 (merged) file support/start-gitlab-docs-nanoc was moved to support/gitlab-docs/start-nanoc, but path in Procfile.erb was not updated to reflect that. This MR simply updates that one line.

To test this change, one simply has to rebuild Procfile and restart gitlab-docs:

make Procfile
gdk restart gitlab-docs

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.