openapi: Fix ‘narrow’ parameter schema.
openapi: Fix ‘narrow’ parameter schema.

The previous schema incorrectly prohibited the two-element array form
that we do in fact accept, and didn’t specify anything about the
contents of the object form.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

zulip/zulip,zulip/zulip,zulip/zulip,rht/zulip,zulip/zulip,rht/zulip,andersk/zulip,zulip/zulip,andersk/zulip,andersk/zulip,rht/zulip,rht/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,rht/zulip,andersk/zulip