Fix schema for room forget call

https://spec.matrix.org/v1.16/client-server-api/#post_matrixclientv3roomsroomidforget fails to define a request body.

But it is not one of the four exceptions listed in https://spec.matrix.org/v1.16/client-server-api/#api-standards which deviate from the rule that 

> All POST and PUT endpoints, with the exception of those listed below, require the client to supply a request body containing a (potentially empty) JSON object.

data/api/client-server/leaving.yaml

@@ -102,6 +102,13 @@ paths:
           example: "!au1ba7o:matrix.org"
           schema:
             type: string
+      requestBody:
+        content:
+          application/json:
+            schema:
+              type: object
+              example: {}
+        required: true
       responses:
         "200":
           description: The room has been forgotten.