[sitemap] increase the DOWNLOAD_WARNSIZE
[sitemap] increase the DOWNLOAD_WARNSIZE

The default value for the DOWNLOAD_WARNSIZE setting is 33554432 (32MB).
This patch will double the value. This solves the following warning:

---snip---
WARNING: Received (37889579) bytes larger than download warn size
(33554432).
---snap---

Change-Id: I5be4bcefa6fe2eb474901bb958669978dd6e1fef

openstack/openstack-doc-tools,openstack/openstack-doc-tools