api/swagger: add missing 400 response for POST /containers/{id}/wait

The /containers/{id}/wait can return a 400 (invalid argument) error if
httputils.ParseForm() fails.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

api/swagger.yaml

@@ -7018,6 +7018,10 @@ paths:
                   Message:
                     description: "Details of an error"
                     type: "string"
+        400:
+          description: "bad parameter"
+          schema:
+            $ref: "#/definitions/ErrorResponse"
         404:
           description: "no such container"
           schema: