Update the expat version referenced in libbsdxml.3

This change updates the version of expat referenced from 2.6.0 to 2.6.3,
correcting documentation post-ffd294a1f4c23863c3e515d16dce31d5509bcb01.

Bump .Dd for the change.

MFC with: ffd294a1f4c23863c3e515d16dce31d5509bcb01
Differential Revision: https://reviews.freebsd.org/D46865