vmstat: fix duplicate key in libxo output

In the libxo output from vmstat, the number of pages that have been
paged out uses the same key name as the number of times pages have been
paged. Appears to have been a typo or copy-pasto.

PR: 222198
Submitted by: Yavuz Tanriverdi <stemix@gmail.com>
Reviewed by: phil, garga
Differential Revision: https://reviews.freebsd.org/D12395