Update DB docs file to match to_yaml syntax

What does this MR do and why?

Update db/docs/approval_group_rules.yml, that was introduced with !132651 (merged), to match the syntax which will be produced by calling to_yaml when re-generating it.

Syntax not matching leads to diff like the following after gdk update:

diff --git a/db/docs/approval_group_rules.yml b/db/docs/approval_group_rules.yml
index 35f753384803..b9dab08c5df4 100644
--- a/db/docs/approval_group_rules.yml
+++ b/db/docs/approval_group_rules.yml
@@ -1,9 +1,9 @@
 ---
 table_name: approval_group_rules
 classes:
-  - ApprovalRules::ApprovalGroupRule
+- ApprovalRules::ApprovalGroupRule
 feature_categories:
-  - source_code_management
+- source_code_management
 description: Keeps approval group rules
 introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/132651
 milestone: '16.5'

The reason to re-generate the file is that it has a model that not exists yet (ApprovalRules::ApprovalGroupRule) which triggers this check here - https://gitlab.com/gitlab-org/gitlab/-/blob/53dcaa7326b5c00d4d2af7dc7bb81cbf898ed114/lib/tasks/gitlab/db.rake#L565. I assume this model will by added shortly, so decided to leave it rather than removing it.

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