Update yaml with organization_id sharding key

Why

The last step in Define a sharding key for snippet_repositories (&19735) epic.

What

• update db/docs/snippet_repositories.yml to set the sharding key:

gitlab_schema: gitlab_main_org
sharding_key:
  snippet_project_id: projects
  snippet_organization_id: organizations

MR: Define snippet repo sharding key (!207525)