extattr_get_file(20: bump .Dd

This is being done for the formatting and context changes. While the net content
hasn't been changed, the content/context changes were sufficient to warrant the
date bump.

MFC after: 1 week
MFC with: r368431, r368433, r368434, r368435
Sponsored by: DellEMC Isilon

(cherry picked from commit 00107a56e597aa029756d1a83321a06e5fc98c23)