[api] pop statistics now returns a JSON object instead of an array with one k/v pair in it, '"cohorts": array-returned-before'.

paths/population_statistics.yaml

@@ -47,6 +47,9 @@ get:
       content:
         application/json:
           schema:
+            type: object
+            properties:
+              cohorts:
                 type: array
                 items:
                   type: object