fix openapi spec to pass swagger editor validation (#4445)

There shouldnt be a dash before `type: object`. Also added description.

pageserver/src/http/openapi_spec.yml

@@ -386,6 +386,7 @@ paths:
         "202":
           description: Tenant attaching scheduled
         "400":
+          description: Bad Request
           content:
             application/json:
               schema:
@@ -945,7 +946,7 @@ components:
               type: string
               enum: [ "maybe", "attached", "failed" ]
             data:
-            - type: object
+              type: object
               properties:
                 reason:
                   type: string