Fixed pageserver openapi spec properties reference (#3904)

## Describe your changes

In [this linter
run](https://github.com/neondatabase/cloud/actions/runs/4553032319/jobs/8029101300?pr=4391)
accidentally found out that spec is invalid. Reference other schemas in
properties should be done the way I changed.

Could not find documentation specifically for schemas embedding in
`components.schemas`, but it seems like the approach is inherited from
json schema:
https://json-schema.org/understanding-json-schema/structuring.html#ref

## Issue ticket number and link
-

## Checklist before requesting a review
- [x] I have performed a self-review of my code.
- [ ] ~If it is a core feature, I have added thorough tests.~
- [ ] ~Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?~
- [ ] ~If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.~

pageserver/src/http/openapi_spec.yml

@@ -873,13 +873,9 @@ components:
       type: object
       properties:
         tenant_specific_overrides:
-          type: object
-          schema:
-            $ref: "#/components/schemas/TenantConfigInfo"
+          $ref: "#/components/schemas/TenantConfigInfo"
         effective_config:
-          type: object
-          schema:
-            $ref: "#/components/schemas/TenantConfigInfo"
+          $ref: "#/components/schemas/TenantConfigInfo"
     TimelineInfo:
       type: object
       required: