device_add_child.9: remove self-reference

Change .Xr to .Nm, which quiets a mandoc warning

Reviewed by: mhorne
MFC after: 3 days
Pull Request: https://github.com/freebsd/freebsd-src/pull/1135

(cherry picked from commit f9ea7b6e8253273ebb1023c4380a305e6d3cf02b)